I think Portuguese has the same concept of definite and indefinite article like the other Romance languages, but I could be mistaken. The definite article ("the") is used to refer to the exact object you are referencing. You are saying there is only one object that matches what you are talking about. The indefinite article ("a" / "an") is used to refer to the object you are referencing as one of multiple possible matches to what you are talking about. Using your example... "She's a teacher" means that there are lots of teachers out there, and this woman you are talking about is one of them. "She's the teacher" means that you have been talking about one specific teacher and the woman is that same, exact person. "My sister went to school for a long time and now she's a teacher." Your sister can now be listed as a teacher with all the other teachers. "There is an ESL class down at the civic center for Portuguese speakers. My sister is the teacher." Your sister is the one and only teacher for this one specific class.

"A teacher" means you're referencing a new subject. "The teacher" indicates she's the teacher you were already talking about. If you were talking about a teacher, then said someone else was "a teacher", then the listener would know that there are two different teachers. If you were talking about a teacher and you said "the teacher", the listener would know that you're talking about the same teacher.

Bom a diferença está no artigo usado... No primeiro caso vc usou o artigo definido "the", e essa frase pode ser traduzida como "Ela é a professora" A segunda frase está com o artigo indefinido "a" e traduz-se "Ela é uma professora" Espero ter ajudado!

Não é verdade, Giovane. Writing contractions such as 'She's' is fine. These forms are part of standard written English.

Cuidado com o uso do apóstrofo, não é muito bom usar contrações na escrita "She's" é o jeito como falamos, quando escrevemos o correto é usar "She is"