首先应该有流派的概念，在大部分情况下，word group 和 phrase 其实是等同的，如：
因此，word group 和 phrase 的概念区分只有在 Systemic Functional Grammar(SFG) 的框架下讨论才是有意义的。这个概念其实没有多少提及的必要，只是胡壮麟本显然受到 SFG （尤其是Hallidy） 相当大的影响。
如果看Halliday的原文，可以发现他给 group 和 phrase 作了如下定义：
对于 group 而言，" ... a group is in some respects equivalent to a word complex that is, a combination of words built up on the basis of a particular logical relation" ," ... that is to say, a Head word together with other words that modify it".[4]
这段话的意思便是，词组是（中心）词的扩展。
Examples:
Look at those two splendid old electric trains with pantographs! (nominal group)
Someone has been eating my porridge. (verbal group)
You’ve coped beautifully tonight; you’ve coped so well compared to how I would have coped. (adverbial group)
对于 phrase 而言，只存在 prepositional phrase，而且prepositional phrase 只有一种基本形态，就是 preposition + noun group. (A prepositional phrase consists of a preposition plus a nominal group. [5])
Examples:
Stop for lunch and a swim at Kuta.
Because literature is in some cases the product of the imagination, isn’t it?
But I can run faster [than a crocodile].
Across the Atlantic, Benjamin Franklin engaged in well-known experiments [with that curious phenomenon, electricity].
Further Readings:
Grammar: Groups & phrases: Prepositional phrases
Nominal group (functional grammar)
THE UNIT "GROUP"
注：看贴上来的原文似乎是中文版，英语语言学的课本尽量看英文版的。
参考1与参考2取自Wikipedia，学会上外网找资料。
