Learn Portuguese (Portugal): Tu não foste ao cinema ontem? - Didn't you go to the cinema yesterday?

You have to rely on context! Because ir (to go) and ser (to be) share the exact same forms in the simple past tense, foste can mean either "you went" or "you were." Here, the phrase ao cinema (to the cinema) implies movement to a destination, making it perfectly clear that we are using the verb ir.

Why is there no Portuguese word for "did" in "não foste"?

Portuguese does not use "helper" verbs like "do" or "did" to form questions or negative statements. To say "didn't you go", you simply take the past tense verb for "you went" (foste) and put não directly in front of it.

Why do we say "ao cinema" instead of just "a cinema"?

The verb ir (to go) is followed by the preposition a (to). Because cinema is a masculine noun, you also need the masculine word for "the", which is o. In Portuguese, you cannot leave a and o separate; they must squish together to form the contraction ao (to the).

Could I drop the word "Tu" and just start the sentence with "Não foste"?

Yes, absolutely! In Portuguese, the ending of the verb usually tells you exactly who the subject is. The form foste is completely unique to tu (informal "you"), so dropping the pronoun is very common and sounds completely natural.