conjunction
for, because
A1
Denn is a coordinating conjunction meaning 'for' or 'because' to give a reason. Unlike subordinating conjunctions it does not change word order (main clause order remains). It can also appear as a modal particle in questions or emphasis (Wie denn?). Use it to link clauses without moving the verb.
Examples
Er nahm den Zug, denn er musste früh ankommen, obwohl das Wetter schlecht war.
He took the train because he had to arrive early, although the weather was bad.
Ich esse, denn ich habe Hunger.
I'm eating, for I am hungry.
Er blieb zu Hause, denn er war krank.
He stayed at home because he was ill.
Details
Mnemonics
Imagine a question and answer: 'Warum? – Denn ...' linking reason.
Sounds like English 'den' with an extra n — think causal connector.
Notes
'Denn' is a coordinating conjunction introducing a reason; it does not change word order.