in the bed:表示在床里面,即在被窝里.例：
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"
in bed 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.例：
Do I have to stay in bed all day?我必须整天呆在床上吗?Reading in bed is bad for your eyes.躺在床上看书对你的眼睛有害.
on the/one’s bed表示“在某张/某人的床上”
例：
She saw a new red box on her bed.在她的床上,她看见一个新的红盒子.
Her skirt is on the bed.她的裙子在那张床上（放着）.