when
The German word 'wenn' is a conjunction that is used to introduce a conditional clause. It is equivalent to the English word 'when' or 'if'. For example, 'Wenn ich Zeit habe, gehe ich ins Kino.' translates to 'When I have time, I go to the cinema.' or 'If I have time, I go to the cinema.'
If it rains, I stay at home.
This sentence is an example of a conditional sentence, where 'Wenn' stands for 'if'. The action in the main clause ('I stay at home') is dependent on the condition in the clause introduced by 'Wenn' ('it rains').
I go running if I have time.
In this sentence, 'Wenn' is used to introduce a condition for the action in the main clause. Here the speaker only goes running when the condition ('if I have time') is met.
Can you please call when you arrive?
This sentence uses 'Wenn' to indicate a time at which the action in the main clause should be performed. It combines a request ('Can you please call') with a condition for performing this request ('when you arrive').