at work 介词短语,英语中介词短语可以作表语.
She is at work.她在工作.
They are at table.他们在吃饭.
He is still in school.他还在上学.
动副短语 就是动词加副词 put up take of turn on 等等
介宾短语 就是介词加 名词,代词,或数词.in red,at work,at six
这是按照组成结构分类,做什么成分,和这些无关.
按性质分：
名词性词组,the teacher‘s desk,Yellow River
形容词性词组,a little ,a few a lot of
动词词组,take care of
副词性词组,at six,at a high speed,
连词词组,neither...nor...both...and...even of...
介词词组 in front of instead of
名词性短语起名词的作用,以此类推.