This week we are going to take a look at the differences between dann and denn, a fairly difficult topic even for experienced non-native German speakers. Part of the problem lies in the fact that in Old High German and Middle High German, these two currently distinct words were just one word, thus they both come from the same root. If we reduce dann and denn to their basic grammatical forms, it's possible to get a better understanding of how they are used.

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1. As an adverb, dann is usually translated as "then":

Wenn die Klassen größer werden als 15 bis 20,

When the classes get bigger than 15 to 20,

dann fange ich an zu teilen.

then I start to split them.

Caption 67, Strothoff International School - Interview mit dem Rektor

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Dann ist auch die Mutter damit beschäftigt, Nahrung herbeizuschaffen.

Then the mother is also occupied with providing nourishment.

Caption 31, Alpenseen - Kühle Schönheiten

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If you ever hear a native German say denn as an adverb in sentences similar to the above, that's because in Northern German dialect, denn and dann are still interchangeable. According to Duden, any other use of denn as an adverb is only very rare. 

2. As a particle, denn is usually translated as "then":

Und was ist denn los?

And what is happening then?

Caption 45, Das Lügenbüro - Die Bewerbung

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Wo wollen wir denn hin?

Where do we want to go then?

Caption 30, Die Klasse - Berlin '61

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Note that the particle denn is almost exclusively found in interrogative sentences (sentences that ask a question)!

3: As a coordinating conjunction, denn is usually translated to English as "because" (or sometimes "since"):

Wir erleben Emotionen und heben ab, denn wir sind frei.

We experience emotions and lift off, because we are free.

Caption 12, Christina Stürmer - Fieber

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Denn plötzlich wurden wir eine richtig moderne Familie.

Because suddenly we became a really modern family.

Caption 21, Mama arbeitet wieder - Kapitel 1: Alle haben sich lieb

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Notice that denn is a conjunction like und or aber or oder and does not force the sentence order to place the verb at the end of the sentence. 

So to recap:  
1. dann as an adverb usually translates as "then";
2. denn as a particle is usually found in sentences asking a question and usually translates as "then," though as an emphasis word it may not be translated at all;
3. denn as a conjunction usually translates as "because," except in
4. Northern German dialect, where denn is used interchangeably for the standard German adverb dann.

Perhaps the easiest way to remember the basic gist of this is: if you want to say "then" in a sentence, it's usually dann; if you want to say "then" in a question, it will most often be denn; or if you want to say "because" you can use denn. This is somewhat oversimplifying the situation, but should serve as a good way to sort out the basics of the differences between dann and denn!

In next week's lesson, we will learn about the difference between the conjunctions denn and weil, both ways to say "because."

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Further Learning
Go to Yabla German and search for dann and denn to see the words used in a real-world context. For an in-depth analysis of the origin of the words dann and denn, read this DWDS page under Etymologie, and be sure and click below it for the full version of the text! For a somewhat lighter-hearted view on the two words, check out this German language blog.