in the bed的意思是在床上，表示在床里面，即在被窝里。in bed的意思是卧床，表示躺在床上、卧病在床，短语中的bed不指具体的床，故前在不用冠词，in也不能改成on。

in the bed造句：

Particles swim around in the bed。颗粒在床层中到处游动。

There seems to be something moving in the bed。床里面好像有个什么在动。

Bubble size is a key factor in determining the conditions in the bed。气泡尺寸是决定床内情况的一个关键因素。

in bed造句：

Mary's in bed，Don't wake her。玛丽在床上睡觉，不要吵醒她。

Jerry often lies in bed and watches TV。杰瑞经常躺在床上看电视。

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