When we talk about time, we can use both “wenn” and “als” in appendage sentences. Both words are translated as “when”, but they are not interchangeable and you need to know exactly when a verb is used.
“Als” is only used as “when” if we’re talking about an action that happened once in the past.
- Als ich dort war, sah ich sie nicht – when I was there, I didn’t see it.
You could say, “ich sah sie nicht, als ich dort war.” Same meaning, difference in order of words. “Als ich dort war” is an appendage sentence where the union “als” always puts the verb in last place. “Ich sah sie nicht” is Hauptsatz (main sentence). When the main sentence is not in the first place in complex sentences, the verb is placed in the first place.
Why did we use the past tense of Präteritum and not Perfekt in our example? If an event occurred once in the past, it most likely takes the form of a narrative of history from the past that is no longer relevant in the present. This is where Präteritum is used.
“Wenn” is used in all other cases: multiple action at any time, single action in the present and future.
- Wenn ich dort war, sah ich sie – (always) when I was there, I saw it.
Obviously, the action has happened many times in the past, because we used “wenn,” not “als. We can even add “always” in our minds.
- Wenn ich dort bin, (dann) sehe ich sie – when I go there, (then) I see it.
Same sentence, only in the present time. So it’s relevant to the past (I’ve always seen her there before) and the future (if I go there, I’ll see her).
- Wenn ich nach München komme, esse ich eine Bratwurst – when I come to Munich I will eat a fried sausage.
And remember not to confuse “wenn” with “wann”. The difference is that “wann” is used in question sentences, and “wenn” in complicated sentences.
- Wann hast du Zeit? – when do you have the time?
- Wenn du Zeit hast, (dann) treffen wir uns – If you have the time, (then) we will meet.
That’s all you need to know about these two unions.