Vi devus ripozi hodiaŭ.

Devus is the -us form, which often gives a softer, less absolute meaning: should / ought to / would be advisable to.

• Vi devus ripozi hodiaŭ. = You should rest today.
• Vi devas ripozi hodiaŭ. = You must have to rest today.

So devus sounds like advice or recommendation, while devas sounds stronger and more necessary.

The ending -us is often called the conditional form, but in everyday use it also commonly expresses:

• something hypothetical
• something polite
• a suggestion
• advice

In this sentence, devus means something like would be supposed to or more naturally in English, should.

So the idea is not that the person is absolutely required to rest, but that resting is the advisable thing.

The ending -i marks the infinitive, the basic dictionary form of a verb.

• ripozi = to rest

After words like devas, devus, povas, volas, Esperanto normally uses the infinitive directly:

• Mi volas manĝi. = I want to eat.
• Ŝi povas veni. = She can come.
• Vi devus ripozi. = You should rest.

So ripozi is there because it is the action being recommended.

In Esperanto, the infinitive ending -i already includes the idea of English to.

So:

• ripozi = to rest
• manĝi = to eat
• iri = to go

That is why Esperanto does not need an extra word before ripozi here.

Vi can mean:

• you singular
• you plural
• formal you
• informal you

Esperanto does not normally distinguish these in everyday standard usage. So Vi devus ripozi hodiaŭ could be said to one person or to several people, depending on context.

Hodiaŭ means today. It is an adverb of time, telling you when the action should happen.

So:

• Vi devus ripozi hodiaŭ. = You should rest today.

It modifies the whole action ripozi.

Yes, Esperanto word order is fairly flexible, especially with adverbs like hodiaŭ.

All of these are possible:

• Vi devus ripozi hodiaŭ.
• Hodiaŭ vi devus ripozi.
• Vi hodiaŭ devus ripozi.

The most neutral version here is probably Vi devus ripozi hodiaŭ.

Changing the position can slightly change emphasis:

• Hodiaŭ vi devus ripozi. = emphasizes today
• Vi devus ripozi hodiaŭ. = more neutral

Not exactly. It is usually understood as advice, not a direct command.

• Vi devus ripozi hodiaŭ. = You should rest today.
• Ripozu hodiaŭ! = Rest today! (a command / imperative)

So devus makes the sentence gentler and less forceful than an outright order.

A rough pronunciation guide for English speakers is:

• Vi ≈ vee
• devus ≈ DEH-voos
• ripozi ≈ ree-POH-zee
• hodiaŭ ≈ roughly ho-DEE-ow

A few helpful pronunciation points:

• v is like English v
• r is usually rolled or tapped
• z is always like the z in zoo
• aŭ is a diphthong, roughly like ow

Stress in Esperanto normally falls on the second-to-last syllable:

• deVUS? Wait carefully: de-vus → stress on DE
• ri-po-zi → stress on PO
• ho-di-aŭ → stress on DI

So the sentence is stressed like this:

• VI DE-vus ri-PO-zi ho-DI-aŭ

The -n ending usually marks a direct object or sometimes motion toward something.

In Vi devus ripozi hodiaŭ, there is no direct object:

• vi is the subject
• devus is the finite verb
• ripozi is an infinitive
• hodiaŭ is an adverb

Since nothing here is a direct object, no -n is needed.

Yes, you could, but the meaning changes.

• Vi devus ripozi hodiaŭ. = You should rest today.
  Advice, recommendation, gentle suggestion.

• Vi devas ripozi hodiaŭ. = You must rest today.
  Stronger obligation, necessity.

So if you want to sound caring or advisory, devus is often the better choice.

Yes. The verb is ripozi = to rest, and the noun is:

• ripozo = rest

For example:

• Vi devus ripozi hodiaŭ. = You should rest today.
• Vi bezonas ripozon hodiaŭ. = You need rest today.

This is a very common Esperanto pattern:

• -i for the verb
• -o for the noun

Yes, absolutely. It is a natural, standard sentence.

It sounds like normal advice someone might give if another person is tired, sick, stressed, or overworked. It is simple, grammatical, and idiomatic Esperanto.