Dann vs. denn
The German words dann and denn are often confused by learners due to their similar spelling. However, they have completely different meanings and grammatical functions. Dann is about time and sequence, while denn is about reason and cause.
Dann
A1The word dann is an adverb of time that means then or afterwards. It is used to describe a sequence of events or a logical consequence. When dann starts a main clause, it typically forces the verb to come immediately after it (in the second position).
Zuerst putze ich die Zähne, dann gehe ich ins Bett.
(First I brush my teeth, then I go to bed.)
Wenn es regnet, dann bleiben wir zu Hause.
(If it rains, then we will stay at home.)
Wir haben gegessen und sind dann spazieren gegangen.
(We ate and then went for a walk.)
Er hat lange überlegt, dann hat er geantwortet.
(He thought for a long time, then he answered.)
Du willst nicht mitkommen? Dann gehe ich eben allein.
(You don't want to come along? In that case, I'll just go alone.)
Denn
A2The word denn is a coordinating conjunction that means because or for. It is used to introduce a main clause that gives a reason or explanation for the previous clause. Unlike other conjunctions like weil, denn does not change the standard word order of the sentence it introduces. It sits in position zero. It can also be used as a modal particle in questions to express surprise or interest, similar to then in English questions.
Ich kann heute nicht arbeiten, denn ich bin krank.
(I cannot work today, because I am sick.)
Er nimmt einen Schirm mit, denn es sieht nach Regen aus.
(He is taking an umbrella with him, for it looks like rain.)
Wir müssen jetzt los, denn der Film fängt gleich an.
(We have to go now, because the movie is about to start.)
Was machst du denn hier so spät?
(What are you doing here so late then?)
Wie alt bist du denn geworden?
(So how old did you turn then?)
Summary
In short, the key difference is meaning and grammar. Use dann (then) to talk about what happens next in a sequence. Use denn (because/for) to give a reason. Remember the grammatical clue: dann often puts the verb in the second position, while denn leaves the word order of the clause unchanged.