Wann vs. wenn
The German words wann and wenn can both be translated as when in English, which often causes confusion for learners. However, they are not interchangeable and are used in different contexts. Wann is a question word for time, while wenn is a conjunction for conditions or recurring events.
Wann
A1The word wann is an interrogative adverb. It is used exclusively to ask about a specific point in time, both in direct and indirect questions. It always translates to when? and asks at what time? or on what day?.
Wann beginnt der Deutschkurs?
(When does the German course begin?)
Ich weiß nicht, wann der Zug ankommt.
(I don't know when the train arrives.)
Wann hast du Geburtstag?
(When is your birthday?)
Er fragte mich, wann ich Zeit hätte.
(He asked me when I would have time.)
Sag mir bitte, wann wir uns treffen.
(Please tell me when we are meeting.)
Wenn
A2The word wenn is a conjunction with two main uses. First, it introduces a conditional clause, translating to if. Second, it introduces a temporal clause, translating to when or whenever for actions that are repeated (in the past, present, or future) or for a single event in the present or future.
Wenn das Wetter schön ist, gehen wir spazieren.
(If the weather is nice, we go for a walk.)
Ruf mich an, wenn du ankommst.
(Call me when you arrive.)
Immer wenn ich jogge, höre ich Podcasts.
(Whenever I go jogging, I listen to podcasts.)
Wenn ich mehr Geld hätte, würde ich ein Haus kaufen.
(If I had more money, I would buy a house.)
Meine Mutter war immer froh, wenn wir sie besuchten.
(My mother was always happy whenever we visited her.)
Summary
In short, the key difference is function. Use wann when you are asking a question about time (at what time?). Use wenn to state a condition (if) or to describe a recurring or future event (when or whenever). A simple test: if you can replace when with at what time? in your English sentence, the German word is wann. Otherwise, it is most likely wenn.