Learn Spanish (Spain): Debes descansar.

In Spanish, when you use deber (to must/should) with another verb, the second verb stays in its infinitive form (like descansar) and attaches directly to it. You do not need any linking words like a or que in between, unlike phrases such as tener que (to have to).

Does debes mean "you must" or "you should"?

It can mean both depending on the context! While it translates literally as "you must", it is very commonly used in the present tense to give strong advice, which translates more naturally to "you should" in English. If you want to soften the advice even more, you could use the conditional form deberías, but debes is perfectly normal for everyday suggestions.

Why isn't the word tú (you) included at the beginning of the sentence?

In Spanish, the ending of the verb tells us who is doing the action. Since debes ends in -s, it can only mean "you" (informal singular). Including tú is not necessary unless you want to add special emphasis to the person you are talking to.

What is the difference between saying debes descansar and tienes que descansar?

Both express an obligation, but they have a slightly different feel. Debes descansar sounds more like strong advice or a moral obligation ("you should rest"). Tienes que descansar (using tener que + infinitive) sounds more like a hard requirement or an unavoidable necessity ("you have to rest").