왜냐하면(wae-nya-ha-myeon, waenyahamyeon)
A conjunctive adverb used to explain reasons
When to use
Reason at sentence start, Presenting logical grounds, Detailed reason in spoken Korean
The nuance
왜냐하면 is used at the beginning or middle of a sentence to explain reasons in detail and is usually followed by a complete sentence.
때문에(ttae-mun-e, ttaemune)
A postpositional particle and conjunction indicating cause or reason
When to use
Cause after noun, Explaining result, Formal sentence structure
The nuance
때문에 attaches after a noun to indicate cause or reason and is mainly used in the middle or end of sentences.