Formation: it's = it is

Usage: It's a beautiful day.

Meaning: It is a beautiful day.

Formation: it's = it has

Usage: It's been really warm the last two weeks.

Meaning: It has been really warm the last two weeks.

Formation: it'll = it will

Usage: It'll probably rain next week.

Meaning: It will probably rain next week.

Formation: it'd = it had

Usage: I'd rained the entire week.

Meaning: It had rained the entire week.

Formation: it'd = it would

Usage: It'd be nice if the sun came out.

Meaning: It would be nice if the sun came out.

Formation: it would've = it would have

Usage: It would've been nice if the sun had come out.

Meaning: It would have been nice if the sun had come out.