Learn French: Tu ne dois regarder que ton livre, sinon tu es exclu. - You must look only at your book, otherwise you are expelled.

In this context, sinon translates to 'otherwise'. It acts as a conditional connector that introduces the consequence of not doing what was just stated. If the person does not look only at their book, the consequence is being expelled. It can also often mean 'if not'.

Why is there a ne and que in this sentence but no pas?

This is the French structure for expressing 'only'. Instead of using a negative like ne ... pas, you frame the verb with ne ... que. The que always goes directly in front of the specific thing being limited, which is why it sits right before ton livre (your book).

Why is there no French word for 'at' after regarder?

In English, we say 'look at', but in French, regarder is a direct verb. It inherently means 'to look at' or 'to watch' and takes a direct object without needing an extra preposition. You simply say regarder ton livre without adding à or de.

If the person being spoken to (tu) is female, does exclu change?