1. "guilty of serious crime" 'guilty' is an adverb. 'of serious crime' another adverb describing guilty. Together they make an adverbial phrase: He was found guilty of serious crime.
2. "the little girl next door" 'next door' is a location. So here we have and adverb of location telling us more about the little girl. This a noun phrase, cf 'next door's little girl'
3. "about what you said" about is a preposition. This is a PP adverbial: About what you said, can you say more?
4. "a good reason why they failed us" 'reason' is a noun here. 'why they failed us' an appositive relative clause . 'a good reason why they failed us' is a noun: (A good reason why they failed us) is (they were drunk). Why they failed us? (A good reason ) is (they were drunk).
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