
als
than
Used for comparisons after a comparative, for example bigger, faster, better.
Mein Bruder ist älter als ich.
My brother is older than me.
than / when / as
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than
Used for comparisons after a comparative, for example bigger, faster, better.
Mein Bruder ist älter als ich.
My brother is older than me.

when
Introduces a subordinate clause that describes a one-time event or state in the past.
Als ich ein Kind war, gab es hier noch keine Autobahn.
When I was a child, there was no highway here yet.

as
Indicates a function, role, or characteristic.
Sie arbeitet als Ärztin im Krankenhaus.
She works as a doctor in the hospital.

as if
Introduces an unreal comparative clause; describes something that does not correspond to reality.
Er tut so, als ob er alles wüsste.
He acts as if he knew everything.

both ... and
A two-part conjunction that lists and connects two equivalent things or situations.
Das Museum ist sowohl für Kinder als auch für Erwachsene interessant.
The museum is interesting both for children and for adults.
After a comparative form, such as 'größer' (bigger), 'besser' (better), or 'schneller' (faster), 'als' (than) is always used for the comparison. The nouns or pronouns being compared stand in the same case.
Ein Elefant ist viel größer als eine Maus.
An elephant is much bigger than a mouse.
As a temporal conjunction, 'als' (when) introduces a subordinate clause that describes a one-time event in the past. The conjugated verb stands at the end of the clause.
Als ich jung war, gab es noch keine Smartphones.
When I was young, there were no smartphones yet.
To describe the profession, role, or function of a person or thing, German uses 'als' (as) followed by a noun in the nominative. With job titles, usually no article is used.
Er arbeitet seit fünf Jahren als Ingenieur.
He has been working as an engineer for five years.
The phrase 'als ob' (as if) introduces an unreal comparative clause, which usually uses Konjunktiv II. The verb stands at the end. A shorter variant is 'als' + Konjunktiv II verb at the beginning of the subordinate clause.
Er redet so, als ob er der Chef wäre.
He talks as if he were the boss.
The fixed expression 'sowohl … als auch' (both … and) is a two-part conjunction. It connects two equivalent parts of a sentence and means something like 'and also'.
Das neue Restaurant bietet sowohl Fleischgerichte als auch vegane Speisen an.
The new restaurant offers both meat dishes and vegan meals.
After a comparative, for example 'älter' (older) or 'besser' (better), 'als' (than) must be used for the comparison. Using 'wie' (like/as) here is a common mistake in colloquial speech, but it is not correct in standard language.
Dein Koffer ist schwerer als meiner.
Your suitcase is heavier than mine.
if / when
Both words can connect a point in time with an event. „Als“ is typically used with a one-time event in the past, while „wenn“ is used with a one-time event in the present or future. Therefore, use „als“ for a past situation and „wenn“ for an upcoming situation.

Als der Bus gestern kam, fuhren wir sofort los.
When the bus arrived yesterday, we left right away.
Wenn der Bus morgen kommt, fahren wir sofort los.
When the bus arrives tomorrow, we'll leave right away.