when

The time at which something happens.

“A good article will cover the who, the what, the when, the where, the why and the how.”

(interrogative) At what time? At which time? Upon which occasion or circumstance? Used to introduce direct or indirect questions about time.

“Do you know when they arrived?”

At an earlier time and under different, usually less favorable, circumstances.

“He's mister high and mighty now, but I remember him when.”

(relative) At which, on which, during which: often omitted or replaced with that.

“That was the day when the Twin Towers fell.”

(interrogative) What time; which time.

“Since when do I need your permission?”

The time that.

“I recall when they were called the Greys.”

At (or as soon as) that time that; at the (or any and every) time that; if.

“A player wins when [as soon as, or at any time that, if] she has four cards of the same suit.”

During the time that; at the time of the action of the following clause or participle phrase.

“Be careful when crossing the street.”

At what time; at which time.

“I am here till Friday, when [i.e. at which time] I leave for Senegal.”

Since; given the fact that; considering that.

“I don't see the point of putting up Christmas decorations when I am the only person who is going to see them.”

Whereas; although; at the same time as; in spite of the fact that.

“He keeps changing things when the existing system works perfectly well.”

That's enough, a command to stop adding something, especially an ingredient of food or drink.

Expressing impatience. (Compare what.)