which one is used more often? denn or weil?

please explain

thank you!

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Definitely "weil". You will almost never encounter "denn" in spoken German.

"denn" is followed by a main clause, while "weil" is followed by a subordinate clause. "weil" is also the appropriate term when you respond to a question inquiring the reason, for example

Warum hast du den Schirm mitgenommen? (Why did you take the umbrella with you?)
(Ich habe den Schirm mitgenommen), Weil ich dachte, dass es regnen könnte

((I took the umbrella with me), Because I thought it might rain)

Also note that "weil" is a so called Subjunktion, which alters the position of the verb (mostly last position of the sentence)