in bed
in the bed：在床上。
He loves being in the bed with his head on the pillow. 他喜欢把头靠在床上的枕头上。
He slept in the bed. 他在床上睡。
in bed：卧床在床上睡着。
It's time the kids were in bed. 孩子们该睡觉了。
The doctor told me I had to stay in bed. 医生对我说我必须卧床休息。
in the bed指的是睡着了，或者是躺在床上，特别是在床上呆了一阵子了。in bed指的是躺在床上。当和ill连用时候才指“抱病在床”。例子：He is ill in the bed
in the bed和in bed的常见搭配：
in bed 表示“躺在床上、卧病在床”。
in the bed：这里有定冠词修饰，表示在床里面，即在被窝里。
例句:
I was so tired and wanted to sleep in the bed right now.
我太累了，真想马上躺在被窝里睡觉
into bed更多表示嵌入床的这个动作，这个人不一定会睡觉也可能去做其他事情。
例句:
Totally exhausted， he tore his clothes off and fell into bed.
他疲惫至极，扯下衣服，一头倒在床上
