Dann and denn
What is the difference between "dann" and "denn"?
One has an "a" in the middle, and the other one an "e". ;-)
Seriously: They are different words, and they are not interchangeable (at least as far as I can see now).
"Denn" often gives a reason: Ich esse etwas Brot, denn sonst bekomme ich Hunger. (I'll eat some bread because otherwise I'll get hungry). It is a coordinating conjunction. http://www.canoo.net/services/Controller?dispatch=glossary=coordinating+conjunction=1=1 (Source: canoo.net, an excellent grammar site for German grammar)
"Dann" ist most frequently used for a sequence in time: Erst esse ich Brot, dann gehe ich spazieren. (I'll eat bread first, and then I'll take a walk.)
"Denn" can also work as a so-called flavoring particle or modifier (wie soll ich das denn wissen -- how am I supposed to know that??). In this case, it is very difficult to translate -- it just supports the emotional content of the sentence.
Do you have a sentence in mind in which you'd like to use one or the other? Then (Dann!) we can discuss the usage in this particular sentence.
dann = then
denn = 1) because, 2) a modal particle that "softens" a sentence
I think the meaning of "then" and "because" are quite clear so I'll only give example sentences of the modal particle "denn".
A) Was machst du hier? (What are you doing here? - Sounds a bit harsh, doesn't it?)
B) Was machst du denn hier? (What are you doing here ? - It has no meaning nor does it really change the meaning of the sentence, but it does make it sound less harsh)