1 when /ˈwɛn/ adverb
1 when
Learner's definition of WHEN
: at what time
: at, in, or during which
: at or during which time
informal : at a former and usually less successful time
2 when /ˈwɛn/ conjunction
2 when
Learner's definition of WHEN
: at or during the time that
: just after the time that
: at any or every time that
used to say what happens, is true, or can be done in a particular situation
used to ask why or how something is done in the situation that exists
: in spite of the fact that : although
: the time or occasion at or in which
3 when /ˈwɛn/ pronoun
3 when
Learner's definition of WHEN
: what or which time

since when?

see 1since