快速答案
deitar 表示躺下或上床；dormir 表示真正睡着。
可靠的方法是先判断这个词在句子里的功能，再翻译。
每个词的作用
deitar
to lie down or go to bed
Use deitar for putting the body in a horizontal position.
dormir
to sleep
Use dormir for being in the state of sleep.
如何选择
- deitar: Choose deitar if the sentence points to putting the body in a horizontal position.
- dormir: Choose dormir if the sentence points to being in the state of sleep.
例句
Vou me deitar cedo hoje.
I am going to bed early today.
Dormi oito horas.
I slept for eight hours.
常见误区
Going to bed does not guarantee sleep: eu me deitei às dez, mas só dormi à meia-noite.
放到语境中练习
从你的真实生活中选一个句子，写两遍：一次用 deitar，一次用 dormir。如果两个版本都可以，说明意思怎样变化。