The Slav Macedonian language is considered by most Slav Macedonians to be one of the most positive contributions to their separate status. The question of what language Slav Macedonians actually spoke prior to 1945 has caused disagreements among even the most disinterested linguists. The Slav Macedonian dialects made a gradual transition from Serbian north of Skopje to Bulgarian in Eastern Macedonia. Even those who claim that a separate Slav Macedonian language existed before 1945 admit that all these dialects have a very close affinity to Bulgarian.  The elements of distinction between the central Slav Macedonian group of dialects (i.e., the language spoken roughly within the region Prilep-Bitolj-Kicevo-Titov Veles) and western Bulgarian were noticeable but not significant.  Bulgarian linguistic influence was strengthened by intense secular and religious propaganda in the past and by the fact that Bulgarian was the language used by the old Slav Macedonian nationalists.
It was natural that the Yugoslav government regarded a distinct Slav Macedonian language as a bulwark against Bulgarian irredentism. At the same time, it was a very real concession to the Macedonians vis-a-vis the Serbs. The First Assembly of the ASNOM in August 1944 passed a resolution declaring Slav Macedonian the republic's official language. A commission was created to determine which features of the spoken dialects were to be incorporated in the written language, and in May 1945 an alphabet was adopted by law.
As the basis for the new literary language the Central Vardar Macedonian dialect was chosen. The explanation was that this region was the most populous area and that it was important in Slav Macedonian history. In addition Krste Misirkov, an advocate of the creation of a separate Slav Macedonian language in the early twentieth century, and other Macedonian nationalists used the central dialects.  However, this dialect is also the Slav Macedonian dialect most unlike both Serbian and Bulgarian. This was probably a far more important consideration in the government's decision. It has been claimed that, in fact, the north-western Slav Macedonian dialects (those most similar to Serbo-Croatian and most unlike Bulgarian) had been originally chosen as the basis for the new language but had to be abandoned because of popular opposition.
From the very beginning, Slav Macedonian linguists concentrated on showing the Slav Macedonian language to be different from other languages. The first grammar, published in 1946, established nine distinctive traits of the new language and stressed its differences from other Slavic languages. At first the language had many words, especially political, literary, philosophical, and technical terms, which were borrowed from Bulgarian, Serbian and Russian. However, from the beginning an effort was made to purge these foreign elements, particularly those from Bulgarian. The commission which codified the language was guided by the principle:
The vocabulary of the literary language should be enriched with terms taken from all Slav Macedonian dialects. New words should be created with living inflections of the folk speech. Borrowed words from other languages should be retained only where necessary, 
As a result, Bulgarian, Russian (after 1948) and other foreign words were replaced by words existing in one of the local Slav Macedonian dialects or by terms created by combining native elements.
In addition to making the modern Slav Macedonian, literary language different from Bulgarian" Yugoslav' linguists also have gone to great effort in attempting to show that the old Slav Macedonian dialects were essentially a separate language. The Slav Macedonian (anti-Bulgarian) interpretation of their linguistic development is not considerd by party leaders to be an ivory tower matter; the campaign is carried on through the mass media. Numerous articles on the alleged historical differences, some dating back to the middle ages, are carried by newspapers. In an early pamphlet (1950) on this subject, the party expressed concern over the popular failure to acknowledge the separate existence of a distinct Slav Macedonian language in the past:
We are meeting with blind acceptance of Great Bulgarian theories with regard to the struggle of our people for its national language. Study of the struggle for a national language is important, because language is one of the four basic elements without which there cannot be talk of a nation. The broad reading public, and even some teachers of national history, know little about the one hundred year struggle of our people for its own national language, or they underestimate it or misunderstand it, and thus they unconsciously become bearers of an anti-historical, unscientific stand with regard to our language. Some lecturers and publicists of our history are bringing to the masses the conception that the Slav Macedonians started in their schools and in their other cultural institutions with the use of the Bulgarian language as their literary language and that this language was used during the whole 19th and 20th centuries until the thirties, when progressive Slav Macedonian publicists took as their literary language one of the Slav Macedonian dialects. Thus it is made to seem that the Slav Macedonian literary language appeared as a shot out of the dark, because with such a conception the struggle for a national language, which started in the middle of the 19th century, is ignored.
This pamphlet then cites several of the historical examples of the use of a Macedonian language, including that "little masterpiece of political agitation," a manifesto made in the name of the ill-fated Krusevo Republic of 1903. The Slav Macedonian Communists do not deny that the language of Goce Delcev and most other national heroes was Bulgarian, or that Bulgarian was the written Slav Macedonian language generally until the Second World War. But Slav Macedonian linguists explain that the Macedonian and Bulgarian peoples were facing essentially the same conditions in their struggle for national liberation against Turkey therefore; they coordinated their efforts. Since the Bulgarian bourgeoisie was more advanced than the Slav Macedonian bourgeoisie, the Bulgarian literary language was further developed; hence, it was used by the Macedonians. The bulk of Slav Macedonian linguistic history is aimed at magnifying the few historical instances of the written use of Slav Macedonian dialects.
Much was done from the beginning to secure wide usage of the new language. The first grammar was published in 1946, an orthography in 1951. An 80,000 word dictionary was published in three volumes between 1961 and 1966. Two journals were started to encourage the use of the Slav Macedonian language-Makedonski jazik (1950) and Literaturen zbov (1954). The new literary language was employed from the very beginning by the mass media of the republic. However, in the early years there were many difficulties in securing wide usage of the still-changing Slav Macedonian language. Writings of the old Slav Macedonian revolutionaries and often even speeches and articles by party leaders had to be translated or adjusted before being used. The lack of adequate language standards and of experience in using the norms that had evolved resulted in linguistic hodgepodges in composition and speech. The vast majority of the population spoke the Slav Macedonian dialects of their regions and the new Slav Macedonian literary language only gradually penetrated the natural speech habits of a population just beginning to pull itself out of the depths of illiteracy and isolation.
A decade after the war, special institutes had to be held for Slav Macedonian language instructors. Commenting on the use of the new Slav Macedonian in literature in 1952, Lunt wrote:
Many Slav Macedonians have not yet learned to use their native Macedonian on all stylistic levels.... It is only the small group of intellectuals daily concerned with the written word who now write easily, without frequent Serbisms of Bulgarisms.... The writers were burdened by their education in Serbian or Bulgarian; they had learned in the long hard years of school Slav Macedonian was only for intimate friends and the most familiar ideas and feelings, but in broader spheres Serbian (or Bulgarian) must be used. This means that even today many Slav Macedonians unconsciously slip into Serbian when discussing political, philosophical or artistic matters. 
The standardization of the new literary language has been a continuing process. But with its constant use in schools, the press, radio, books and theater, Slav Macedonians have gradually come to understand and use the new language. The major non- Slav Macedonian cultural pull, because of party pressure, has been to Serbo-Croatian rather than Bulgarian. Serbo-Croatian is the second language in Slav Macedonian schools. As the output of original Slav Macedonian literary works and even translations of standard Communist works into Macedonian was modest in the beginning, Serbo-Croatian was widely read. In time the supply of Slav Macedonian textbooks, manuals and propaganda pamphlets has improved. Bulgarian books were discouraged before 1948 and prohibited for a time after the Cominform break. Although at times when Bulgarian-Yugoslav relations have been good, Bulgarian works have been available throughout Yugoslavia, their accessibility in Macedonia has always been more limited.
It is not surprising that the output of Slav Macedonian literature has been limited both in quantity and quality. This is natural in view of the relative newness of the Macedonian language, the availability of Serbo-Croatian literature, and the fact that secondary linguistic problems are only now being solved. The early postwar literary efforts were limited primarily to poetry and a few short stories. As the language has become more firmly established and as writers have become accustomed to using it, literary works have increased both in volume and quality. Of those literary figures who have appeared, none can be said to be towering.
It is doubtful if the impact of Slav Macedonian literature as such on the development of nationalism will prove as powerful as the use of the new language in the mass media and increasingly in everyday speech. In addition to the difficulty of developing a separate literature for a group of one million people in the shadow of the well-established Serbian and Croatian literatures, the literary themes called for are not always such as will encourage a "nationalist" literature. Immediately after the war the main themes were the partisan struggle with emphasis on the brotherly struggle of all Yugoslav peoples as a precondition for Slav Macedonian liberation. Although since 1948 there has been a shift away from the sterile black-and-white Soviet style of "socialist realism," Nova Makedonija and the Macedonian party leadership have continued to exhort writers to deal with "progressive" subjects. A campaign in the early 1950's opposed literary concentration on folklore and encouraged treatment of universal Marxist motives. Works based on Macedonian national characteristics are frowned upon, but still produced.
Reports on Slav Macedonian acceptance of the language have varied greatly. Some emigres claimed that the efforts to de-Bulgarize the language led to the inclusion of so many foreign elements that almost all Slav Macedonians instinctively reject it. One Serbian specialist in Yugoslav Macedonia in the early 1950's reported that the new Macedonian was used mostly by government employees loyal to Belgrade. However, one British expert on Macedonia held the language to be "immensely popular," and another expressed uncritical enthusiasm about the "happy" acceptance of it and the other new Slav Macedonian cultural forms. A more realistic assessment comes from a Skopje schoolteacher who emigrated from Yugoslavia:
Among the wide masses of the urban population and the intelligentsia, the Slav Macedonian language is accepted as the most important, and often the only good, aspect of the present day Yugoslavia. The children are learning it in the schools and their parents are very satisfied that this is the case. There are places in the country-side where people were reluctant to send their children to school in the days of old Yugoslavia, but now they do so willingly, for they want their children to learn the Slav Macedonian language....The new literature and poetry in the national language has aroused great interest, for through it is created and formed the new national spirit and language. This new literature, as well as the printing of prewar literature and poetry by Slav Macedonians in the national language, has resulted in much reading.
Having been taught in schools and used extensively throughout the SRM for over twenty-five years, the Slav Macedonian language is accepted by most Slav Macedonians.
The concern of the Slav Macedonian and Yugoslav Communist leaders whenever the existence of the Slav Macedonian language is questioned reflects their feeling that the language is one of the principal elements of a separate Slav Macedonian national consciousness. In 1958 several Bulgarian statements declared that the Slavic inhabitants of Macedonia spoke Bulgarian, not the "semi-Serbian literary language which is fabricated in Skopje." Lazar Kolisevski, defensively answering the Bulgarian claims, denied the "alleged 'Serbianization'" of the Slav Macedonian language but justified the frequent use of Serbian expressions:
The Slav Macedonian language cannot be isolated from the mutual influence of the languages spoken by the Yugoslav peoples. Our common socio-economic development and socialist practice have created and are creating a number of new general expressions and terms accepted by all Yugoslav peoples.... The development of the languages of nations which have appeared late on the stage of history provides numerous examples showing that they are subjected to the influence of more developed languages and richer national cultures of related and neighboring peoples. 
Denial of the existence of the Slav Macedonian language is considered so serious a challenge to the Slav Macedonian nationality that Belgrade has not hesitated to condemn the Bulgarians regardless of the state of relations with Sofia. Vigorous and vehement denunciations of Bulgarian academicians have been published by leading Yugoslav newspapers even during periods of good relations with Bulgaria. The Yugoslav leaders thus acknowledge that the wide, if imperfect, usage of the Slav Macedonian language is one of the most vital contributions to Slav Macedonian nationalism.
*This text is from the book of Yugoslav Communism and the Macedonian Question by Stephen E. Palmer, Jr. Robert R. King, 1971. In order to segregate the Greek Macedonian cultural identity from the Slav one, I add for understating purposes the words Slav and Vardar in the front of the Greek adjective Macedonia (n) at the text. Also in the book you can find the original notes and bibliography that used in the specific article.