it's... = it is... (contraction)
it's... = it has... (contraction)
but different from the two above:
its... = something/somebody of something (possessive pronoun)
It's (= It is) Maggie that broke the window.
It's (= It has) been raining since midday.
This dog is injured. One of its legs (= one of the dog's legs) is bleeding.
Bear in mind I'm not a teacher!
- For Teachers