As an interrogative adverb:
* Quand? (When? - most common translation)
As a conjunction:
* Quand (When - used to introduce a subordinate clause)
* Lorsque (When - more formal, can also mean "as soon as")
* Dès que (As soon as)
* Au moment où (At the moment when)
In other contexts:
* Alors (Then, at that time)
* À l'époque (At the time)
Examples:
* When will you be home? -> Quand seras-tu à la maison?
* I saw her when I was at the market. -> Je l'ai vue quand j'étais au marché.
* When you are ready, we can leave. -> Quand tu seras prêt, on peut partir.
* She arrived when the party was already in full swing. -> Elle est arrivée lorsque la fête battait déjà son plein.
The best translation depends on the specific sentence and the intended meaning.