I / you / he / she / it / we / they + [verb]
A subject pronoun stands for a person or thing that you have already named, or that both people can see. It comes before the verb.
You has one form for one person and for a group.
Use it for one thing and they for more than one.
They is also for one person when you don't know or don't say if it's a man or a woman.
A sentence with a verb needs a subject, even when it's clear who you mean. Only instructions like Sit down. have no subject. For weather, time and distance there's no real subject, so you use it.