As a question word:
* Quand? (most common) - This translates directly to "when?"
* À quel moment? - This means "at what moment?" and is a more formal way of asking.
As a conjunction:
* Quand - This translates directly to "when" and introduces a subordinate clause.
* Lorsque - This means "when" and is often used for a more formal or literary style.
* Dès que - This translates to "as soon as" and implies immediacy.
* Au moment où - This means "at the moment when" and emphasizes the specific moment in time.
As an adverb:
* Quand - This translates directly to "when" and indicates a time.
* Alors - This means "then" and is used to indicate a time after something else.
Examples:
* When will you arrive? - Quand arriveras-tu?
* I will call you when I get home. - Je t'appellerai quand j'arriverai à la maison.
* He left when the bell rang. - Il est parti lorsque la cloche a sonné.
* He was happy when he saw her. - Il était heureux quand il l'a vue.
So, the best translation of "when" will depend on the specific context of the sentence.