when

adverb

• at what time:when did you last see him? [with preposition]:since when have you been interested?
• how soon:when can I see you?
• in what circumstances:when would such a rule be justifiable?

adverb

• at or on which (referring to a time or circumstance):Saturday is the day when I get my hair done

conjunction

• 1at or during the time that:I loved maths when I was at school
• after:call me when you’ve finished
• at any time that; whenever:can you spare five minutes when it’s convenient?

• 2after which; and just then (implying suddenness):he had just drifted off to sleep when the phone rang

• 3in view of the fact that; considering that:why bother to paint it when you can photograph it with the same effect?

• 4although; whereas:I’m saying it now when I should have told you long ago