denn. The verb stays.

denn also means "because." It also takes a comma. But it does the one thing weil never does: it leaves the word order completely alone.

Two words for "because"

German has two conjunctions that translate as "because." They mean the same thing but behave differently. The difference comes down to one question: does the verb move?

Conjunction	Meaning	Verb position
weil	because	Moves to the end of the clause
denn	because	Stays in position 2

weil takes the clause that follows and reorganizes it: verb goes to the end. denn simply connects two complete sentences without touching either of them. Both sides stay exactly as they are.

How weil works: weil. The verb goes last. Another conjunction that also sends the verb to the end: dass. What someone thinks, says, or feels.

The verb stays in position 2

With denn, the second clause reads exactly like a normal sentence. Subject in position 1, conjugated verb in position 2, everything else after.

Main clause	denn	Position 1	Verb (pos. 2)	Rest
Ich lerneDeutsch,	denn	ich	arbeite	in Berlin.
Er kommt nicht,	denn	er	ist	krank.
Sie lernt viel,	denn	diePrüfung	ist	schwer.
Wir bleiben zu Hause,	denn	es	regnet.

With modal verbs: normal order

Because denn does not change the word order, modal verbs behave exactly as they do in any other sentence. The modal sits in position 2 and the infinitive goes to the end.

Main clause	denn	Subject	Modal (pos. 2)	Rest	Infinitive (last)
Ich kann nicht kommen,	denn	ich	muss	heute	arbeiten.
Er bleibt zu Hause,	denn	er	will	das	sehen.
Sie lernt mehr,	denn	sie	muss	diePrüfung	bestehen.

Compare this directly with weil: with weil, the infinitive comes before the modal. With denn, the modal comes before the infinitive. Normal sentence order, every time.

One more difference: denn cannot move

A weil clause can be placed at the start of a sentence. When it is, the main clause simply inverts its subject and verb. denn cannot do this. It always connects two clauses in the same order, with the reason coming second.

	Example	Valid?
weil at the start	Weil ich in Berlin arbeite, lerne ich Deutsch.	✓
denn at the start	Denn ich arbeite in Berlin, lerne ich Deutsch.	✗

Which one to use

Both are correct in most situations. weil is more common in everyday speech. denn sounds slightly more formal and is more frequent in writing. If you are unsure, weil is always the safe choice. Once sending the verb to the end feels natural with weil, start using both and feel the difference.