in the bed的意思是在床上，表示在床里面，即在被窩里。in bed的意思是臥床，表示躺在床上、臥病在床，短語中的bed不指具體的床，故前在不用冠詞，in也不能改成on。

in the bed造句：

Particles swim around in the bed。顆粒在床層中到處游動。

There seems to be something moving in the bed。床里面好像有個什么在動。

Bubble size is a key factor in determining the conditions in the bed。氣泡尺寸是決定床內情況的一個關鍵因素。

in bed造句：

Mary's in bed，Don't wake her?，旣愒诖采纤X，不要吵醒她。

Jerry often lies in bed and watches TV。杰瑞經常躺在床上看電視。

He was lying in bed talking on the phone with his girlfriend。他躺在床上和女朋友打電話。