What is the difference between 'wenn' and 'wann'?

'Wann' is a question word and can only be used to start questions beginning with 'when' or in indirect questions. For example, in the question 'Wann hast du Geburtstag?' (When is your birthday?) or the indirect question 'Ich weiß nicht, wann er Geburtstag hat.' (I don't know when his birthday is.)'Wenn' often corresponds to the English 'if' or 'whenever', and is used when referring to present, future or repeated past actions. For example, 'Wenn ich spät nach Hause komme, bin ich müde.' (If/when I come home late, I am tired.) / 'Wenn ich älter bin, werde ich Arzt sein.' (When I am older, I will be a doctor.)/ 'Wenn ich im Park war, konnte ich mit meinen Freunden spielen.' (When/whenever I was in the park, I could play with my friends.)Note that 'als' can also be used to refer to a single occurrence or a state in the past, which would be translated with 'when'. For example, 'Als ich jung war, hatte ich ein Kaninchen.' (When I was young, I had a rabbit.)