It's or Its? Their, There or They're? A Quick Way to Tell

It's vs. its

It's always means 'it is' or 'it has'. Its shows that something belongs to it. The apostrophe here is a contraction mark, not a possession mark, which is why the possessive has none.

The test: say the sentence with 'it is'. If it still makes sense, use it's. If not, use its.

It's raining outside. (It is raining outside.) The dog wagged its tail. ('It is tail' makes no sense, so no apostrophe.)

Their, there and they're

Their shows ownership by more than one person. There points to a place or introduces a fact. They're means 'they are'.

Their house is on the corner. The keys are over there. They're coming at six.

The test for they're is the same as before: replace it with 'they are'. If the sentence still works, you need the contraction.

Your and you're

This pair works exactly like it's and its. You're means 'you are'. Your shows that something belongs to you.

You're welcome to join us. Your ticket is at the front desk.

Who's and whose

Who's is short for 'who is' or 'who has'. Whose asks about ownership.

Who's coming to dinner? Whose jacket is this?

Why spellcheck often misses these

Every one of these is a real word, so a basic spellchecker sees nothing wrong. Only a tool that reads the whole sentence can tell that 'its raining' is off. That is why a final read-through, out loud if possible, still matters.