A quick hit:
It's = It is
Its = A possessive pronoun, e.g. 'The cat licked its paw.'
This gets so commonly misused that it's practically becoming acceptable to use It's instead of Its. A neat way to avoid the trap is to remind yourself that It's is always two words. When in doubt, say the two words in full, e.g. "The alien protected it's eggs." The alien protected it is eggs? I don't think so.
They're = They are
Their = Possessive pronoun
Your = Possessive pronoun
You're = You are
These are the most common offenders in the scripts that I read. It's disconcerting. You may think I'm being pedantic but when you read a script with no typos or grammar discrepancies, it makes a huge difference, and the writer or writing stands out with more distinction.