Conjunction

1. At what time.

   They were told when to sleep.

2. At such time as.

   I’m happiest when I’m working.

3. As soon as.

   I’ll do it when I get the time.

4. At a time in the past,

   It was raining when I came yesterday.

Adverb

1. At what time.

   When will they arrive? (direct question)

   Do you know when they'll arrive? (indirect question)

Pronoun

1. What time; which time

   Since when do I need your permission?

Noun

1. The time.

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

Interjection

1. Said to indicate that an implied condition or time has been reached (used for synchronization). Similar to "now!"; often used with an undertone of whimsy.

   Anne (pouring milk): Say "when". Bob (indicating that enough milk has now been poured): When!