"Dann" is an adverb that means something is happening after something else has happened ("then", "after that" in English). "Denn" is a main clause causal conjunction that means "because". Ich bin aufgestanden. Dann habe ich mir die Zähne geputzt. = I got up. Then I brushed my teeth. Ich bin nass geworden, denn es hat geregnet. = I got wet, because it rained. Note the difference in word order: The adverb "dann" takes the first place (the verb "habe" follows in second place), while the conjunction "denn" is between sentences, and the subject "es" takes the first place (and "hat" follows in second place).