Here's how to tell:
* Subject Pronoun: When "it" is the one performing the action in a sentence.
* Example: It is raining. ("It" is the thing doing the raining)
* Object Pronoun: When "it" is receiving the action in a sentence.
* Example: I saw it flying. ("It" is the thing being seen)
Important Note: The word "it" can also be a demonstrative pronoun when it refers to something specific mentioned earlier.
* Example: I saw a bird, and it was blue. ("It" refers to the bird mentioned earlier)