
weshalb
why
Used as an interrogative adverb to ask about the reason or cause of something. Synonymous with 'warum' (why).
Weshalb hast du mich nicht früher angerufen?
Why didn't you call me earlier?
why / which is why
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why
Used as an interrogative adverb to ask about the reason or cause of something. Synonymous with 'warum' (why).
Weshalb hast du mich nicht früher angerufen?
Why didn't you call me earlier?

which is why
Used as a relative adverb to introduce a clause that gives the reason for the action or state mentioned in the main clause.
Der Flug hatte Verspätung, weshalb wir den Anschlusszug verpassten.
The flight was delayed, which is why we missed the connecting train.
'Weshalb' (why / which is why) can introduce a question or connect two main clauses. In both cases, 'weshalb' stands at the beginning of the sentence (position 1), and the conjugated verb comes directly after it (position 2). It does not introduce a subordinate clause with the verb at the end.
Weshalb ist der Laden heute geschlossen?
Why is the shop closed today?
'Weshalb' (why / which is why) is often perceived as somewhat more formal or more typical of written language than the more common 'warum' (why). In spoken language, 'warum' is more usual.
Der Kunde war unzufrieden, weshalb er eine Beschwerde einreichte.
The customer was dissatisfied, which is why he filed a complaint.
The word is made up of 'wes-' (an old genitive form of 'was' (what)) and '-halb' (in the sense of 'because of'). So it literally means 'because of what' or 'for what reason'.
Weshalb bist du so fröhlich?
Why are you so cheerful?
why
Warum and weshalb both ask for a reason, and both are possible in a direct question. Weshalb can additionally introduce a relative clause about the reason, especially in the phrase „der Grund, weshalb“. For a normal question, either word works; for this relative construction, weshalb is typical.

Das ist der Grund, weshalb der Zug heute im Bahnhof stehen bleibt.
That is the reason why the train is staying at the station today.
Warum bleibt der Zug heute im Bahnhof stehen?
Why is the train staying at the station today?