sit in
1．sit in a place，for a special purpose为某一特殊目的而坐在某处
He sits in with the patients while the nurse has a rest．护士休息的时候，他坐在那儿照看病人。
2．replace临时代替
The president is ill tonight so the secretary will be sitting in for her as chairman at the meeting．主席今晚生病，因此秘书将代她主持会议。
3． demonstrate by sitting in a place坐某处示威
The students have been sitting in for the last two weeks．上两个星期学生一直在静坐示威。
4．babysit替人临时照看婴孩
When her son and daughterin一law went to the cinema，she had to sit in with their child．她儿子、儿媳去看电影时，她得替他们照看孩子。