So you're a bit confused about the difference between "their", "there" and "they're", or "your" and "you're", or "its" and "it's"?
Well you're not alone; many people have trouble with this, not least because they're often taught badly at school by teachers who themselves don't take the time to understand the relatively simple rules. However the rules really are quite consistent once you know them, and not too hard to remember:
Always use "it's" (rather than "its") immediately before:
Usually use "its" (rather than "it's") immediately before any sequence of adjectives followed by a noun.
The structure of these words is a hang-over from very early English, when the order of words was less significant and the case markings were used to indicate the purpose of each word.
Nowadays the order of words is pretty much fixed, except when pronouns are used, which is why we still use the old-fashioned case markings for them. In fact, it's even more important to get the case markings on pronouns right because we no longer use case markings on any other words.
Try out the combinations for yourself
Just in case the above rules aren't clear, here's the full table...
|I||we||you||he||she||it||they||who||who is being talked about?|
|my||our||your||his||her||its||their||whose||who(m) does the thing mentioned next belong to?|
|mine||ours||yours||his||hers||its||theirs||whose||who(m) does the thing previously mentioned belong to?|
|who is the subject of the action?|
|who is being talking about, and what are they doing?|
|who is being talking about, and what did they do?|
Apostrophes originally always meant an abbreviation of some kind. Today its primary use is still to indicate that some letters have been removed, but sometimes those letters have been gone so long that people often don't realise they were ever there.
Many centuries ago the usual way to create a possessive was simply to add "es" to the end of the word. (A possessive is the form of a word that indicates a relationship with some other word, such as "my" in "my dog" or "dog's" in "the dog's bone".)
If thou pullest the cates tail, it will turn and bite thee.
Note that "cates" was pronounced as two syllables. Most words followed this pattern, but the pronouns (I, thou, he, she, it, we, you and they) were much older still and had their own possessive forms (my, thy, his, her, its, our, your and their).
As the centuries wore on the second syllable became compressed until it no longer had a vowel sound, and the spelling adapted by putting an apostrophe in its place. That is still the case today. There were complications of course: "s's" sounds exactly like "s", so the second "s" has been dropped; and some words already ended with an "e" which is silent and hence not removed but the apostrophe is still put there for consistency.
Besides possessives, apostrophes are still used in abbreviations, notably negations like "can't" ("can not") and "won't" ("will not"), and in odd places such as "o'clock" ("of the clock"). Note that "want" (meaning "desire") is not an abbreviation, and so doesn't have an apostrophe.
In summary, the same rule for apostrophes has always applied to possessives as to everything else, but the pronouns never had "e" to begin with, so they never have an apostrophe to indicate that it's gone away.
Jennifer Crusie has an interesting take on the use of punctuation when writing fiction, where the viewpoint of the character and the precise expression of timing and emphasis is more important than slavish following of rules. But you have to know what the rules are before you can know why you're breaking them!