when

/wen/

at what time

When will the meeting start?

Asking about the time the meeting begins

general

at what time or point in time

I'll call you when I arrive.

The action of calling will happen at the time of arrival

When did you last see her?

Asking about the specific time of the last encounter

Often used in questions and conditional clauses

general

if or at the time that

When it rains, the ground gets wet.

Describing a conditional relationship between events

When you're ready, we can leave.

Expressing a condition for an action

Used to introduce conditional clauses

Collocations

when in doubta phrase meaning to do something safe or conventional if you're unsurewhen all is said and doneafter everything has been considered or completedwhen push comes to shovewhen a difficult decision must be made

do as the locals do

when someone in charge is not present

'When' is commonly used to form questions about time, e.g., 'When did you leave?'

'When' can introduce conditional clauses, often meaning 'if' in some contexts.

Old English 'hwænne', from Proto-Germanic 'hwan', meaning 'at what time'

Can function as an adverb (asking about time) or conjunction (introducing conditional clauses). Often used in questions and conditional statements.

English Dictionary