you should always remember to knock the door when

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.
再问： 为什么不能填enter？？
再答： enter 是动词，

entering 是现在分词
在when后要填现在分词。