Learn Portuguese (Portugal): Nós falamos à noite.

The grave accent indicates a contraction. It combines the preposition a (at/to) and the feminine article a (the). Because noite is a feminine word, we use à to mean "at the".

I have seen de noite before. Can I use that instead of à noite?

Yes, both mean "at night" and are often interchangeable. à noite is very common for saying "in the evening" or "at night" as a general time of day. de noite usually emphasizes the state of it being nighttime or dark out.

How can I tell if falamos means "we talk" (present) or "we talked" (past)?

In Portugal, the present tense is falamos (no accent), and the past tense is written with an acute accent: falámos. Because there is no accent here, you know for sure it means "we talk" right now or as a routine.

Can I leave out the word Nós?

Yes, you can just say Falamos à noite. The verb ending -amos already tells us that the subject is "we". Portuguese speakers often drop the pronoun unless they want to add emphasis.