angenommen
assuming
Used as a particle to introduce a hypothetical situation or assumption; in the sense of 'supposing that...'.
Angenommen, wir verpassen den Zug – was machen wir dann?
Assuming we miss the train — what do we do then?

assuming / accepted
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assuming
Used as a particle to introduce a hypothetical situation or assumption; in the sense of 'supposing that...'.
Angenommen, wir verpassen den Zug – was machen wir dann?
Assuming we miss the train — what do we do then?

'Angenommen' is the past participle of the verb 'annehmen'. This verb has two main meanings: 'to suppose/think hypothetically' and 'to accept something'. The two uses of 'angenommen' follow directly from these meanings.
Ich nehme an, du kommst morgen zur Party.
I assume you're coming to the party tomorrow.
You can recognize the meaning from the sentence position. If 'Angenommen' (assuming) is at the beginning of the sentence, followed by a comma, it introduces a hypothesis. If it appears before a noun, it functions as an adjective and describes that noun's accepted state.
Angenommen, das Wetter ist schlecht, dann grillen wir drinnen.
Assuming the weather is bad, we'll grill indoors.
When 'Angenommen' (assuming) introduces a hypothesis, it stands in position 1. After that comes a comma and a normal main clause, with the verb in position 2.
Angenommen, du gewinnst im Lotto, was würdest du kaufen?
Assuming you win the lottery, what would you buy?
In the meaning of 'accepted', the word is often used in formal or administrative contexts. Typical combinations are 'ein angenommener Antrag' (an accepted application/motion), 'ein angenommenes Gesetz' (an adopted law), or 'ein angenommenes Angebot' (an accepted offer).
Nach der Abstimmung galt der Antrag als angenommen.
After the vote, the motion was considered accepted.
if / when
'wenn' and 'angenommen' can both introduce a condition or an imagined situation. 'wenn' is the normal conjunction for real or possible conditions: if something happens, something else follows. 'angenommen' is used when you deliberately present a situation only as an assumption. It often sounds like: Let’s imagine that ... Learners may confuse the words because both can come before a condition. 'wenn' fits in almost all normal conditional sentences; 'angenommen' fits better when you are starting an example or a thought experiment.

Angenommen, der Zug fällt aus, dann fahren wir mit dem Bus.
Assuming the train is canceled, we’ll take the bus.
Wenn ich die Prüfung bestehe, feiere ich mit meinen Freunden.
If I pass the exam, I’ll celebrate with my friends.