Both mean because.
I know that denn does not change the sentence structure of the subordinate clause, but weil does, i.e. pushing the verb to the end.
Other than that, is there a difference between the two? Are there situations where weil is preferred to denn and vice-versa?
The differences between "denn" and "weil" are syntactic only.
Another notable syntactic difference is that you cannot begin a sentence with a main clause introduced by "denn", while the equivalent with "weil" is well possible. Compare:
Er nahm einen Schirm mit, weil es stark regnete.
Weil es stark regnete, nahm er einen Schirm mit.
Both these sentences are grammatical. However:
Er nahm einen Schirm mit, denn es regnete stark.
*Denn es regnete stark, nahm er einen Schirm mit.
Here, the first sentence is fine, but the second is not.
Semantically, there is no difference between "denn" and "weil". You will find that "weil" is much more commonly used in spoken German.
I disagree that they are 100% synonymous.
I my experience, denn is more common when the causality is less necessary, and the point of the clause is to introduce helpful, but possibly more parenthetical information.
Weil, by contrast, tends to imply a more strictly necessary condition.
For example, I might say:
Ich habe auf ihn geschossen, weil es der Oberst mir befohlen hatte.
but it sounds less exculpatory to say:
Ich habe auf ihn geschossen, denn der Oberst hatte es mir befohlen.
Also, weil allows you to add qualifiers, like nur. I'm not sure you can say nur denn, and if you can, it sounds weird.
There are further differences between denn and weil, see for instance http://www.ling.upenn.edu/~tatjana/papers/scheffler-AC05.pdf
Here is a helpful example of when you can use "denn" but "weil" doesn't really make sense:
Er muss müde sein, denn er trinkt viel Kaffee.
"He must be tired, because / seeing as he is drinking a lot of coffee."
versus
Er muss müde sein, weil er viel Kaffee trinkt.
"He must be tired, because / reason being he is drinking coffee."
"Weil" implies the coffee is causing him to be tired, as if it is the origin of his state of tiredness.
It is a rare situation to run into, but this example at least shines a little light on the difference between the two.
SOURCE: Handbuch zur Deutschen Grammatik Sixth Edition
The same distinction exists between for/because in English.
"Denn" corresponds exactly in function and meaning to the archaic English conjunction "for", which was common in early modern English:
"Blessed are the meek, for they shall inherit the earth."
"Father, forgive them, for they know not what they do."
"Weil" is a subordinate conjunction equivalent to "because".
"I was absent because I was sick".
The semantic meaning is approximately the same, except that the latter more strongly specifies causation. The grammatical function is different. Weil/because introduce subordinate clauses, which is important to remember in German because they have a different word order.
Just learning German, but I stumbled upon this issue in my studying, and I am wondering if this little test of sorts is helpful:
If you are joining two statements A and B,
Use denn if it makes sense to say “I’m saying ‘A’, because B is true.”
Use weil if it makes sense to say “A is true because B is true.”
The difference is purely grammatical.
Denn is a coordinating conjunction and takes place 0 in German word order.
Weil is a subordinating conjunction and can start a sentence and is known as a "kicker" and therefore kicks the conjugated verb out of second position and to last.
Denn may not begin a sentence either, if starting the sentence with "because" in German always use weil.
Die Webseite deutsche Sprache, schwere Sprache hat eine sehr schöne Erklärung dafür:
Beispielsatz:
"Ich werde diese Schallplatte nicht kaufen, weil sie zerkratzt ist."
Die Regel:
Der Beispielsatz ist richtig. Weil steht immer mit Verb-Letztstellung. Entscheidet man sich für einen Nebensatz mit weil, muss das Verb am Ende des Nebensatzes stehen (Verb-Letztstellung).
Beispielsatz:
"Ich werde diese Schallplatte nicht kaufen, denn sie ist zerkratzt."
Denn steht hingegen mit Verb-Zweitstellung. Dabei steht das Verb an zweiter Stelle des Nebensatzes, wie in einem Hauptsatz.
Von der Bedeutung her sind "weil" und "denn" gleich und werden mit "because" ins Englische übersetzt. Im Deutschen muss der Satzbau angepaßt werden.
I've been asked that before by exchange students. I couldn't come up with any examples where the meaning would be different; checking several on-line resources didn't give any results either.
A noteworthy side note might be that it becomes more and more common for native speakers to use the denn grammar for the weil sentence while adding a short pause after the weil, which sounds just horrible sometimes.
Not an answer, but just a remark: It is no longer mandatory that WEIL pushes the verb to the end. Like with any other reform, some Germans find this disgusting and there is some controversy, for example, this text is funny:
http://www.spiegel.de/kultur/zwiebelfisch/zwiebelfisch-weil-das-ist-ein-nebensatz-a-350013.html
Another, not completely off-topic, remark: when speaking German with natives (or at least when trying...) I have understood why there must be inversion for some particles and why not for others. It helps the listener distinguish the particle itself (and so the meaning of the whole sentence).
For instance, you may easily confuse denn and wenn when hearing a person's voice in the middle of the street. But the inversion helps distinguish between them:
Anne geht nicht spazieren, denn sie ist krank.
Anne geht nicht spazieren, wenn sie krank ist.
If "wenn sie ist krank" were right, you would easily confuse both sentences. (It is an off-topic remark, but I think it is interesting and, up to some degree, related)
I don't know why they did that reform for the use of weil. For me, it sounds horrible without inversion...
I keep reading the difference is "denn" introduces a main clause. That is bad Grammar. Both words are subordinating conjunctions, which turn a clause into a subordinate clause. A clause can always stand alone as an independent sentence when not having a conjunction. Coordinating conjunctions are used to join two independent clause which involve no causality. Subordinating conjunctions create causality. If there is a definitive usage difference between "denn" and "weil" outside of the German language having multiple linguistical sources (various dialects being welded together), it has never been clearly defined, even in German Grammar. There is just "the one sounds right." The only correct issue I have read here is that one does not start sentences with "denn"- although, colloquially, I have heard that more and more.
