Its is a possessive pronoun.
Ex: This is the dog's house. It's its house.
It's has two possibilities:
1) A contraction of it and is
Ex: It is its house / It's its house.
2) A contraction of it and has
Ex: It has been a long time / It's been a long time.
Note, with it's, look for a present participle; e.g., been. If you see one, you'll know that it's stands for it has, not it is.
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