This is a very difficult point for non portuguese speakers because most languages have the verb 'to be' with the same meaning either for SER (É) or for ESTAR (ESTÁ). The main difference is: É (verbo SER) is used for things that have permanent status and ESTÁ (verbo ESTAR) is used for things that have temporary status. That's why 'o taxi está' and 'o banheiro é'. This is what you have to know by now. For instance, I can say "Eu sou professor", that's what I do. I couldn't say "Eu estou professor" in a regular conversation, but it has a metaphorical meaning if I want to say that this is not my profission, I'm just doing this temporalily. However, it is too specific and you must not worry about that now. Remember only the difference I told you at first.

Try to think in "está" more like a situation, a moment, a state. Like, in this moment you can "be" sad, but you're not sad ALL the time in your life, so in this example you say: "Eu ESTOU triste" and not "eu SOU triste"

That's a simple question. A taxi can move, a bathroom can't. That's why we use estar and ser respectively.