It's 2 words, put a space on there people. A lot.

Never write "should of".

You always mean should have.

The reason why people write this mistake sometime is because "have" and "of" can sound the same.

have, have, should have.

I should have done that.

That is not "should of" it is "should have".

I should have done that.

Its If you can replace what you're saying with

it is, use an apostrophe.

If you can't, don't use an apostrophe.

It's cold.

It is cold.

Use an apostrophe.

Its closure is magnetic.

It is closure.

No, that doesn't make sense.

Its.

Without an apostrophe.