Learn Portuguese (Portugal): Queres sair hoje à noite?

This is exactly how the modal verb querer (to want) works! When you want to do an action, you conjugate the first verb (queres = you want) and leave the second verb in its full infinitive form (sair = to go out). You never conjugate the second verb in this kind of pairing.

Where is the word for 'you' in this sentence?

In Portuguese, you can usually drop the subject pronoun if the verb makes it obvious who is doing the action. The ending of queres tells us it is the informal 'you' (tu). You could say Tu queres sair, but dropping the tu is much more common and sounds very natural.

Why is 'tonight' translated as hoje à noite?

Portuguese does not have a single exact word for 'tonight'. Instead, you say hoje (today) plus à noite (at night), literally meaning 'today at night'.

What does the backwards accent on the à in à noite mean?

The word à with a grave accent is a contraction. It combines the preposition a (at/in) and the feminine word for 'the', which is also a. So à noite literally means 'at the night'. The accent is just a visual clue that those two identical little words crashed together into one.