If you are going to criticize others' spelling, learn to use proper grammar

If you are going to criticize others' spelling, learn to use proper grammar.

Quote: "I understood the movie and it's point which is of course..."

By using "it's" you are saying "it is" and therefore your sentence could read:
"I understood the movie and it is point which is of course..."

The correct version of this sentence should be:
"I understood the movie and its point which is of course"

Your criticism of their spelling is only believable if you can communicate well. The fact that you cannot use proper grammar makes your point less believable.