idiom是成语,习惯用语(我这里牛津版的英语成语词典里面收录的大部分都是我们所说的phrase“词组”)slang是俚语、黑话,通常具有地域性歇后语应该是a two-part allegorical saying 但是英文里的歇后语在中文中不一定有相应的歇后语与之对应,通常是义译出来例如:To kick down the ladder—— To forget favors打掉梯子——过河拆桥A flash in the pan——In like a lion,out like a lamb平底锅里一闪亮——不了了之(对应中文成语虎头蛇尾)TO return to our sheep——To get down to brass tacks再回到羊身上——言归正传,闲话少说As poor as a church mouse——To be down and out穷得像教堂里的老鼠——一贫如洗,家徒四壁To hitch one's wagon to a star——To have a been in one's bonnet把马车扔上星星——吃心妄想,白日做梦As gentle as a lamb——To be nonplussed像羊羔一般有礼——无言以对,悄声无息