In casual conversation, estar is abbreviated like this:
estou ➜ tô
está ➜ tá
estamos ➜ tamos
estão ➜ tão
This is becomming the accepted standard, preferred form of speaking the verb estar. In written Portuguese the non-abbreviated form is always used.
Eu estou aqui. ➜ Eu tô aqui. ➜ I'm here.
Como você está? ➜ Como você tá? ➜ How are you?
Use estar to talk about temporary, passing things.
Eu estou atrasado. (I'm late.)
Onde você está? (Where are you?)
Estamos perto do metrô. (We're close to the subway.)
Eles estão no restaurante. (They're at the restaurant.)
Ser is for somewhat permanent characteristics. Don’t take this too literally – nothing is truly permanent 😉
very permanent ➜ Meu nome é Raquel. ➜ My name is Raquel.
somewhat permanent ➜ Eu sou brasileiro. ➜ I'm Brazilian.)
not so permanent ➜ Ele é casado. ➜ He's married.
Estar is for temporary passing things.
Eu estou cansado. (I'm tired.)
Como você está? (How are you?)
To express was.
When talking about action in the past, use the imperfect indicative tense of estar –> estava.
Eu estava almoçando quando você ligou. ➜ I was having lunch when you called.
Ela estava ocupada ontem. ➜ She was busy yesterday.
*Don’t use this to express was for talking about permanent things in the past (I was a young boy; She was a sick baby etc.) – use ser for that.