als. One moment in the past.

als means "when," but only for one specific moment or period in the past. Not repeated, not ongoing, one time. The verb goes to the end, and als can never mean "if."

The one rule that decides everything

Both als and wenn translate as "when" in English. German keeps them completely separate based on one question: is it a single moment in the past, or anything else?

Situation	Use
One specific moment or period in the past	als
Present or future	wenn
Repeated or habitual (past or present)	wenn
"If" (condition)	wenn

als has exactly one job. Everything else goes to wenn.

Als ich ein Kind war

"When I was a child" describes a single continuous period, your childhood. It happened once. It is in the past and it is over. That is als.

You cannot say "Wenn ich ein Kind war" here. That would imply it happened repeatedly, "every time I happened to be a child", which makes no sense. Childhood happens once. Use als.

More examples with als

als clause	Main clause	English
Als er ankam,	waren wir schon da.	When he arrived, we were already there.
Als sie dieNachricht hörte,	weinte sie.	When she heard the news, she cried.
Als ich 20 war,	zog ich nach Berlin.	When I was 20, I moved to Berlin.
Als dasSpiel endete,	gingen alle nach Hause.	When the game ended, everyone went home.

Each of these describes one event, one moment, one period. It happened once and it is done. That is the signature of als.

The verb goes to the end

Like all the other conjunctions in this series, als sends the verb to the last slot in its clause. The comma is always required. The clause can come first, in which case the main clause inverts its subject and verb.

als clause first	Verb	Subject	Rest
Als ich einKind war,	wohnte	ich	in München.
Als er ankam,	waren	wir	schon da.
Als sie 20 war,	zog	sie	nach Berlin.

The quick test

Ask yourself: is this something that happened once, in the past, and is now over? If yes → als. If there is any chance it repeats, continues, or hasn't happened yet → wenn.

The full picture of wenn: wenn. When or if, the verb goes last. The verb-last rule from the beginning: weil. The verb goes last. For two past events in sequence, see nachdem. The clause that came first.