ph.

1. 介入，干预，插手(棘手问题)
If you step in, you get involved in a difficult situation because you think you can or should help with it.

1. act as a substitute

e.g. She stood in for the soprano who suffered from a cold

Synonym: substitutedeputizedeputise

2. get involved, so as to alter or hinder an action, or through force or threat of force

e.g. Why did the U.S. not intervene earlier in WW II?

Synonym: interveneinterfereinterpose