Learn Italian: Perché lavori senza di noi? - Why are you working without us?

In Italian, the word for "without" is senza. However, when you use senza right before a personal pronoun like noi (us), me (me), or te (you), you must add di. If you were talking about a regular noun instead, you would just use senza by itself, like senza scarpe (without shoes).

Is noi the exact same word used for "we"?

Yes! In English, we change the word depending on its job: "we" for the subject and "us" after a preposition. In Italian, noi does double duty. It acts as the subject ("we") and also as the strong pronoun that comes after prepositions (like in senza di noi).

How do we know it means "are you working" if there is no "you" in the sentence?

The ending of the verb tells us. Lavori ends in -i, which is the standard ending for "you" (singular, informal) in regular -are verbs like lavorare (to work). Because the verb ending makes it completely clear who is doing the action, Italian usually leaves out the subject pronoun tu (you).

Doesn't perché usually mean "because"?

It means both! In Italian, perché is used for "why" in questions and also for "because" in answers. You can easily tell it means "why" here because it is asking a question.