How do I keep als, wenn and wann apart?

Wann asks; als tells you about one past occasion; wenn covers everything else. A short version many learners use: wann for questions, als for once, wenn for whenever. It is crude and it predicts correctly.

Does als have other meanings?

Yes, two worth knowing. It means than in comparisons, größer als ich, and as in a role: als Lehrer arbeiten, to work as a teacher. Neither has anything to do with time, so they will not interfere with this rule.

Is wenn the same as ob?

No. Ob means whether and introduces an indirect yes-or-no question: ich weiß nicht, ob er kommt. Wenn means if in the conditional sense. English uses if for both, which is why they get swapped.

Which one to useB1

Als vs wenn

The short answer

Als marks a single completed event in the past, as in als ich in Berlin wohnte. Wenn covers everything else: repeated events in the past, present habits, and anything at all in the future, as in wenn ich Zeit habe. Wann is neither of those. It is the question word.

At a glance

Als or Wenn?

Compare	AlsOnce, in the past.	WennWhenever, or in the future.
Time frame	Past only	Past, present and future
How many times	Once	Repeatedly, or every time
English	when	when, whenever, if
Typical companion	damals, plötzlich	immer, jedes Mal, jeden Tag
In questions	Not used: that is wann	Not used: that is wann
Conditional sense	Never	Yes: wenn also means if

How to choose

Ask these, in order

1
Is it a one-off event in the past?
Als.
Als ich ankam, war niemand da.
When I arrived, nobody was there.
2
Did it happen repeatedly in the past?
Wenn, usually with immer or jedes Mal to make the repetition explicit.
Immer wenn ich ihn sah, lächelte er.
Every time I saw him, he smiled.
NBSame past tense as als would take, only the number of occasions differs.
3
Is it present or future?
Wenn, always. Als has no present or future use at all.
Wenn ich fertig bin, rufe ich dich an.
When I’m finished, I’ll call you.
4
Are you asking a question?
Wann, neither of the other two.
Wann kommst du?
When are you coming?

Side by side

Each one, in full

Als

When to use als

Once, in the past.

Als introduces one specific occasion in the past, and that is its only job. Als ich klein war, als wir uns kennenlernten, als der Krieg zu Ende ging: each names a single point or period that happened once.

Because it is a subordinating conjunction, the verb goes to the end of its clause: als ich in Berlin wohnte. If the als clause comes first, the main clause then begins with its verb: als ich ankam, war niemand da.

Als ich Kind war, wohnten wir in Hamburg.

When I was a child, we lived in Hamburg.

Sie hat angerufen, als du draußen warst.

She called while you were outside.

Als er das hörte, wurde er blass.

When he heard that, he went pale.

NBOne moment, so als, and the main clause inverts.

Wenn

When to use wenn

Whenever, or in the future.

Wenn covers repetition and futurity, and it also means if. Wenn ich Zeit habe, komme ich can be when I have time or if I have time: German does not distinguish, and context decides.

For a repeated past event, wenn is correct and immer or jedes Mal usually accompanies it to rule out the conditional reading: immer wenn wir dort waren, hat es geregnet.

Wenn es regnet, bleiben wir zu Hause.

If it rains, we’ll stay at home.

Jedes Mal, wenn ich koche, geht etwas schief.

Every time I cook, something goes wrong.

Ruf mich an, wenn du ankommst.

Call me when you arrive.

Mistakes to avoid

What learners actually get wrong

Wenn ich gestern ankam, war niemand da.
Als ich gestern ankam, war niemand da.
A single past event takes als. Wenn would say it happened every time you arrived, which gestern flatly contradicts.
Als ich Zeit habe, rufe ich dich an.
Wenn ich Zeit habe, rufe ich dich an.
Als has no present or future use whatsoever. Anything not firmly in the past takes wenn, however single the occasion.
Ich weiß nicht, wenn er kommt.
Ich weiß nicht, wann er kommt.
This is an indirect question, so it needs the question word wann. Wenn there would mean if he comes, which changes the sentence from not knowing the time to not knowing whether at all.
Als ich in Berlin wohnte war es billiger.
Als ich in Berlin wohnte, war es billiger.
German always puts a comma between a subordinate clause and the main clause, and the main clause then starts with its verb. Both halves of the rule are easy to drop together.

Questions

Asked and answered

How do I keep als, wenn and wann apart?: Wann asks; als tells you about one past occasion; wenn covers everything else. A short version many learners use: wann for questions, als for once, wenn for whenever. It is crude and it predicts correctly.
Does als have other meanings?: Yes, two worth knowing. It means than in comparisons, größer als ich, and as in a role: als Lehrer arbeiten, to work as a teacher. Neither has anything to do with time, so they will not interfere with this rule.
Is wenn the same as ob?: No. Ob means whether and introduces an indirect yes-or-no question: ich weiß nicht, ob er kommt. Wenn means if in the conditional sense. English uses if for both, which is why they get swapped.

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