There are 2 main reasons for using apostrophes:
One or more letters is missed out: can’t (can not)
Together with an “s”, it shows possession: Helen’s coat (the coat belonging to Helen)

Most of you have this down.  Some of you are still forgetting to use any at all or are using them before plural “s” or “s” on a verb:  He write’s; he uses simile’s
This, I have to tell you, is a hideous error that causes us to suck air in rapidly through our teeth (or the less reserved amongst us to swear).  Please do not do this!!!!  I cannot state strongly enough how irritating it is - and, therefore, what a poor impression of your academic abilities it gives.  I know that people’s worth in the “real world” is not measured by their ability to use apostrophes, and I really, honest guv, am not a member of the apostrophe police, but I assure you that if you are studying English at AS/A2 level, you really really ought to be able to punctuate correctly! <rant off>

The sticking point, I think, is with the word “its” - it breaks the rules.  
There are, in fact, two versions: It’s = it is.  Its = belonging to it.
In most cases, its (possessive) occurs in student work referring to the text’s purpose/audience etc.  “Its intention is ....”
The golden rule is: IF you can say “it is” where you have an “Its”, then use the apostrophe.  If you can’t, don’t.  For example: “it is intention is” clearly is gibberish, so it doesn’t need an apostrophe.  Clear?  Good.