Colloquially they are used interchangeably. However, if you wanna impress your German friends, this is what I've found (I didn't even know this myself as a native speaker) on the differences between Warum/Wieso/Weshalb:
Warum is asking for the reasons or motives
- Warum freut sich die Frau? (Why is the woman rejoicing?)
- Weil sie einen rosa Pinguin gewonnen hat. (Because she won a pink penguin)
Wieso is asking for the cause
- Wieso fällt der Pinguin auf den Boden? (Why does the penguin fall on the floor)
- Weil er der Schwerkraft ausgesetzt ist. (Because he is subject to gravity)
Weshalb is asking for the purpose/aim
- Weshalb geht er auf Duolingo.com? (Why does he go on duolingo.com)
- Weil er etwas lernen möchte. (Because he wants to learn something)
AS AN EXTRA BONUS, here is the German song from the Sesame Street, which features all three words: https://www.youtube.com/watch?v=uPHi5xn_q5c
still confused.. What are the differences between reasons and causes? What would you use for "Why are you late for school?" "My bike was stolen" ---- (cause?) "I got up late" ---- (reason?)
I feel that weather it's asking for a reason, a cause or a purpose depends on the answer, not the question... such as "Why do you eat a hamburger for lunch?" "Because it tastes good" ---- (reason) "Because it's fast and I ran out of time" ----(purpose)
Roughly, a reason or a motive is a logical or psychological entity - like a justification, a premise, a propositional explanation etc. A cause is a nomological entity of the natural world - say, a physical or biological law, or a common-sense counterpart of that. "I fell because you pushed me" is a cause. "I fell because I wanted to test the carpet's softness" is a motive or reason. Nevertheless, the boundaries are not always so philosophically clear, as you could find out on Anscombe's, Wittgenstein's and Davidson's discussions about it.
From the Questions 2 - Tips and notes:
"Warum" (why) is also not declinable.
"Wieso", "Weshalb", and "Weswegen" can be used instead of "Warum." There's no difference in meaning.
For example, take "Warum ist das Auto so alt?" = "Wieso ist das Auto so alt?" = "Weshalb ist das Auto so alt?" = "Weswegen ist das Auto so alt?" (Why is the car so old?).
Not sure if this was also in the older version of the German tree. I don't remember seeing weswegen back then.
Steven13579, are you saying that it is hard to know which word to use (Warum/Wieso/Weshalb) because they each provide more information about the answer than the question itself? For example, if I ask "Why were you late for school?" I may not know which word to use because I don't know whether you were late due to external reasons outside of your control (Wieso), internal motivations (Warum), or a specific purpose/aim (Weshalb).
If that is what you meant, I would say that it doesn't matter that you don't know why a particular event (being "late for school") happened. The way you ask the question would only reveal what you expect the reason to have been. So if I use "Warum" to ask why you were late for school, that would show that I think you had an internal reason/motivation for being late (such as "I woke up late."). And if I'm wrong, you could correct me by saying, "My bike was stolen." (external cause)
Does this capture it? Weshalb geht er in den Laden? (Why is he going to the store?) Er geht in den Laden, um Brot zu kaufen (He is going to the store to buy bread)
Wieso geht er in den Laden? (Why does he go to the store?) Weil seine frau ihn gebeten hat? (because his wife asked him to)
Warum geht er in den Laden (Why is he going to the store?) Weil es seine Frau glücklich machen wird (Because it will make his wife happy)
Link for the curious:
I would say that "Warum and Wieso" can be used in whatever you wanna ask. For example: "Warum machst du das"? (Why are you doing that?) Wieso willst du dich nicht beruhigen? (Why don't you want to calm yourself down?) You can also use "Weshalb" in the same way. Like: "Weshalb brauchst du den Bleistift?" (Why do you need the pencil?) Just think about it
Very nice tip about "Why" translation in German: http://blogs.transparent.com/german/wieso-weshalb-warum-%E2%80%93-why/