you should always remember to knock the door when _____ the room.

问题描述:

you should always remember to knock the door when _____ the room.
该填什么?

You should always remember to knock the door when (entering)the room.
进入房间时,你应该总是记得敲门.为什么不能填enter??enter 是动词,

entering 是现在分词
在when后要填现在分词。