The English language can be most confounding at times.
Let's look at just a few examples ... ...
... Take the case of "I" vs. "me", "he" vs. "him", etc. Most of us are taught, from a very early age, to begin a sentence with I (not me), he (not him), etc. And then, of course, comes this result -- when you hear a person say, "I and Pete went to the store." Now they have to be corrected. (You know, it's not polite to list yourself first.) That's confusion enough, I should think.
Where the confusion really sets in is when you want to refer to yourself or the second person in the middle (or even at the end) of a sentence. "It is I" and "Is that he?" are both correct, by the way. I know, I know. Neither 'sounds right'. (One of the reasons they don't sound right is that very few people speak our common language correctly!)
... Let's look at plurals. We are taught for the sake of simplicity that, to indicate a plural, one merely places an "s" at the end of the original word. Then they try to teach us the many exceptions: families/familys, women/womans/, geese/gooses, we/I, they/he, you/you ... WAIT! Where'd that one come from??
[Legend has it here in the south that "y'all" is singular, and "all y'all" is plural.]
ToDAY, tho, I'd like to focus on "its" vs. "it's". In my opinion, this is the most commonly misunderstood and misapplied.
There is one hard and fast rule -- no exceptions. (At least, none to my knowledge!) "It's" is a contraction, a shorter way of saying "It is". (The apostrophe takes the place of the missing letter/s. In this case, the second 'i'.) Let's look at some examples:
1. It's my turn. (It is my turn.)
2. It's about time you came back. (It is about time you came back.)
3. That company should get it's act together. (Did you mean to say, "That company should get 'it is' act together?" Of course not.) Should have said "its act".
I repeat, "It's" is a contraction, a shorter way of saying "It is".
At all other times, including those denoting possession, use "its". Examples:
1. Its purpose is to put gas in your tank. (Correct.)
2. Don't you think it's about time we acted? (Correct ... it is.)
3. Once this happens, its really not much longer until ... (Not correct. You meant to say, 'it is really not much longer until', but you did not indicate the contraction with an apostrophe.)
SO, apostrophe = contraction. No apostrophe = all other applications.
I invite your comments or questions of concern. I will do my very best to answer them in a positive and constructive manner.