Hallo zusammen,
I know they’re not used the same way (with “weil” the verb goes at the end, and with “denn” at the second position).
What I’m not sure of though is: in which context can both “weil” and “denn” be used or when can we only use “weil” (or only “denn” if that exists)?
Thanks
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I would also love to know the answer to this @jeancome! I usually just resort to “weil”.
Looking forward to hearing some responses!
That’s a good question! I do not think that weil and denn have such a big difference. I would say that you use weil when the reason for it is known and you are sure about it.
Er kann gut kochen.
Er ist ein Koch!
Er kann gut kochen, weil er ein Koch ist. ( ich bin mir darüber ziemlich sicher 
)
But if you use denn it may well be the case that you are not sure if something will happen or if the reason is right.
Er kann gut kochen, denn er ist (vermutlich) ein Koch!
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@Mona_O, that sounds similar to “wenn/falls”, but I don’t agree with that (just my opinion, I am not a linguist). For me “weil” and “denn” (and “da”) are synonyms and I use them whether I am sure of something or not. 
I don’t know if one of them was maybe formerly used for known reasons and one for unknown reasons (presenting the person you are talking to with new information). But I really believe nowadays you use them for the exact same meaning. I personally feel that weil is more common for talking and denn for writing. I myself use a lot of denns when I write a (rather formal) text, but I almost never use it in a conversation.
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I agree with you Lydia.
‘Weil’ is used almost all the time when we Germanspeakers talk, ‘denn’ and ‘da’ we hardly use at all in conversations, but more in formal texts.
For me, they have the same meaning.
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I have to admit that I’m not a linguist too 
and after thinking about it for a while, I agree with you both too.
I actually thought the same way you did and found that weil is used very often when speaking, and denn and da are very common while writing.
However, in the case of denn I have always had the feeling that it is used in very formal situations or in unknown or in less known situations 
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IMO there is no real difference between weil and denn - however - the way people use both varies. Like Nils said, denn and da are more for formal texts. Towards that I think it depends on what kind of people you are talking to, you might hear ‘highly educated’ people use denn more often than during a conversation with friends. Also, if you don’t repeat weil over and over again you might want to vary and use words like denn, da, etc…
Just like in English, most people use because but as, since and for are hardly used any more.
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@jeancome sent in an appeal about a sentence. He answered with “Sie ist traurig, denn sie kann nicht bei ihrer Familie sein.” but it was marked incorrect, so it got me thinking about why it sounded wrong to me. I agree with all of the answers here.
By definition denn and weil are identical, but there is a slight difference for me in this case. If you want to give a reason for something (why she’s sad), I would always use weil.
However, if a reason is not evident at first and only becomes clear after something else happens/somebody tells you, you can use denn.
My best example for this:
Fahr vorsichtig, denn die Straßen sind glatt! (Drive carefully, since/because the roads are slippery!)
Die Straßen sind glatt, weil es geregnet hat. (The roads are slippery, because it rained.)
For the German learners here, just go with weil, as that always works 
You’ll come across denn more in written texts.
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Awesome answers! I don’t seem to hear people use denn very often either, but i’m a student here.
Would you guys say that the difference in level of formality between wenn/falls is similar? I feel a little more formal or matter of fact when I use falls speaking German. However, I do notice people use falls often so I do not mean similar in that sense.
I would equate the difference of wenn = if and falls = in the case of
Ich will auch ein bisschen mein Deutsch üben, weil… ich euch auf Deutsch kenne… und ich mag einfach Deutschlernen. 
danke euch allen für die tolle Antworte. Ich höre gar nicht so oft denn in der gesprochene Sprache, aber ich bin Schuler hier.
Würdet ihr sagen, dass der Unterscheid der Formalität zwischen wenn/falls ähnlich wie dieser Fall ist? ich fühle mich ein bisschen, als ob falls sich wie eine bessere Wortwahl anhört, trotzdem meine ich nicht, dass dieses Wort nicht oft benutzt wird.
Ich würde personlich wie das folgende übersetzten wenn = if and falls = in the case of
#correctswelcome
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