in既可作介词又可作副词
adv.
进入,入内
The door being opened,they came in at once.
门一打开,他们就马上进来了.
prep.
(表示位置)在…里面; 在,于; 在…部位上
I could feel the tension in the room.
我可以感觉到房间里的紧张气氛.
They live in France.
他们住在法国.