答案:Compounds differ from free phrases in the following three aspects.1) Phonetic features. In compounds the word stress usually occurs on the first element whereas in noun phrases the second element is generally stressed if there is only one stess. (2 points)2) Semantic features. Compounds are different from free phrases in semantic unity. Every compound should express a single idea just as one word. (2 points)3) Grammatical features. A compound tends to play a single grammatical role in a sentence. (2 points)