In English there is only one word for “when,” but in German there are three words: wann, wenn, and als. In German, it’s very important to use the correct word, otherwise the whole meaning of the sentence can change.

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Wann is a question word used to ask “at what time” directly as a question, as Diane demonstrates:

Und wann läuft der?

And when does it start [run]?

Caption 40, Diane erklärt - Fragewörter

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When you ask a question about the past, you also need to use wann:

Wann hast du Wiener Kurti zuletzt gesehen?

When did you last see "Wiener Kurti" [a nickname]?

Caption 67, Alexander Hauff - Showreel

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Or when you ask indirectly, as Piggeldy does to Frederick when talking about the arrival of summer:

Dann wollen wir mal den Frühling fragen, wann der Sommer kommt.

Then we want to ask spring when summer is coming.

Caption 17, Piggeldy und Frederick - Sommer

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Wenn is the most common form of “when,” referring to time in the following examples:

Wenn er hinter der Bar steht, gibt er alles,

When he is standing behind the bar, he gives it everything,

wie viele seiner Kollegen.

like many of his colleagues.

Caption 34, Cocktails mixen - So „shaken“ die Besten

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When planning a journey, you would use wenn as shown here:

OK, und wenn ich im Europapark bin,

OK, and when I arrive at Europapark,

wo kann ich dann übernachten?

where can I spend the night [stay]?

Caption 21, Reiseplanung - Anruf bei einem Reisebüro

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Wenn doesn’t just mean “when”; in German it is also used to express “if”:

Wir würden uns freuen, wenn alle Menschen auf PETA de [www.peta.de] unsere Onlinepetition unterstützen.

We would be happy, if all people would support our online petition on PETA de [www.peta.de].

Caption 13, PETA-Aktion - Gegen das Wal-Massaker

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And the third word for “when” is als, which is always used to describe an event that has already happened or a single point in time. Wann or wenn just wouldn’t be correct here. Remember this and you’re halfway there!

Und haben Sie schon mal gedacht irgendwann,

And did you ever think, at some point,

als Sie Ihre Fotos gesehen haben...

when you saw your photos...

Caption 30, Bambi-Verleihung - No-Gos auf dem Roten Teppich

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It's also shown here as Angela Merkel talks about a past atrocity:

Als im Herbst eine rechtsextremistische Terror- und Mörderbande aufgedeckt wurde.

When in the fall a right-wing extremist terrorist [organization] was uncovered.

Caption 58, Angela Merkel - Neujahrsansprache

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So as you see, distinguishing between wann, wenn, and als is not so difficult if you remember the basics outlined above!