phrase: is applicable to any group of words that recurs frequently but is likely to suggest some distinctive quality, such as triteness, pithiness, or pointedness: a poem made up of several trite ~s  strung together.      trite陈腐pithy精辟pointed尖锐等没有译出，因为一般phrase仅表示不构成句子的一组词，即短语词组      又如he uses the phrase 'I strongly believe' too often他总是用'我强烈相信'这一短语(词组)


idiom: applies to a combination of word elements which is peculiar to the language in which is occurs either in its grammatical relationships or in its nonliteral meaning: "to keep house", "to catch cold", and "to strike a bargain" are ~s.       idiom习惯用语是某种语言"特有"的短语，可表现为1、特有的非字面含义peculiar nonliteral meaning，如poker face面无表情喜怒不行于色<==扑克脸，如to ride shotgun坐前排<==骑猎枪，如本例to strike a bargain达成交易<==打击减价商品    或2、特有的语法关系peculiar grammatical relationships，如stupid is as stupid does做蠢事才是真的蠢(表示看起来蠢的人未必蠢，摘自阿甘正传)，其他语言不会这么用词造句，因此称为idiom，来自词根IDIO表示自己的私有的one's own, private      普通英语母语者掌握几千个习惯用语，形容词idiomatic表示符合某种语言的特有习惯的，即地道的。说母语者的语言可能显得粗鄙无知没文化，但一定idiomatic地道。相反的，外语学习者即使是科学家教授博士有很高的文化，也很少能达到美国初中生的idiomatic水平。


expression: may be preferred when reference to a way of expressing oneself is accompanied by a qualifying adjective, phrase, or clause: that's an odd ~.  old-fashioned expression老套的表达方式


locution: a somewhat bookish word, may be chosen when reference is to phrases that are idiomatically peculiar to a language, a group, or a person: a pet ~ of the author.


phrase词组短语: 泛指常用的一组词，idiom习惯用语: 某种语言特有的词语组合，具有特殊的语法关系或非字面含义，见注释，expression表达方式表达式: 即用于表达的短语，常接限定性形容词短语从句，locution惯用语（书）: 较书卷气，指某个人或群体或语言所特有的习语


记忆方法: 1）首字母PIEL一堆一摞<==词组

         2）练习一下

         3）词组的意思是共同表达含义、作为句子成分的一组词mean a group of words which together express a notion and which may be used as part of a sentence.