Η ξεκούραση είναι ουσιαστική για την ανάρρωση.

A word-for-word breakdown is:

• Η = the
• ξεκούραση = rest
• είναι = is
• ουσιαστική = essential, important, substantial depending on context
• για = for
• την = the
• ανάρρωση = recovery

So the full sentence means something like Rest is essential for recovery.

Η is the feminine singular definite article, meaning the.

Greek nouns have grammatical gender, and ξεκούραση is a feminine noun. That is why it takes η in the nominative singular:

• η ξεκούραση = the rest

In this sentence, η ξεκούραση is the subject, so it appears in the nominative case.

Because it describes ξεκούραση, which is feminine.

In Greek, adjectives usually agree with the noun they describe in:

• gender
• number
• case

Since ξεκούραση is:

• feminine
• singular
• nominative

the adjective also appears as feminine singular nominative:

• ουσιαστική

You can compare:

• η ξεκούραση είναι ουσιαστική = feminine
• ο ύπνος είναι ουσιαστικός = masculine
• το διάλειμμα είναι ουσιαστικό = neuter

In standard modern Greek, you normally do use the verb είναι for is/are in this kind of sentence.

So:

• Η ξεκούραση είναι ουσιαστική = Rest is essential

Unlike some languages, Greek does not normally drop the present-tense form of to be in neutral standard sentences like this.

Both are possible in Greek, but they do not feel exactly the same.

• για την ανάρρωση = for recovery / for the recovery, referring to recovery in a general but definite sense
• για ανάρρωση can sound more indefinite or more like for recovery purposes, depending on context

In many general statements, Greek often uses the definite article where English may not.

So για την ανάρρωση is very natural here.

Because after the preposition για the noun goes in the accusative case.

So the feminine singular article changes:

• nominative: η ανάρρωση
• accusative: την ανάρρωση

That is why the sentence has:

• για την ανάρρωση = for recovery

This is a very common pattern in Greek:

• για τη δουλειά = for work
• για την υγεία = for health

The two important nouns are in different cases:

• η ξεκούραση → nominative
  because it is the subject of the sentence

• την ανάρρωση → accusative
  because it comes after the preposition για

So the structure is:

• subject: Η ξεκούραση
• verb: είναι
• predicate adjective: ουσιαστική
• prepositional phrase: για την ανάρρωση

A simple pronunciation guide:

• ξεκούραση ≈ kseh-KOO-ra-see
• ανάρρωση ≈ a-NA-ro-see

A few helpful notes:

• ξ sounds like ks
• ου sounds like oo
• ση at the end often sounds like see
• rr in ανάρρωση comes from the spelling ρρ, but in normal speech you do not need to make it dramatically different as a beginner

The sentence stress falls mainly on:

• ξεκούραση
• ουσιαστική
• ανάρρωση

Yes. Greek word order is more flexible than English, although not every version sounds equally neutral.

The most neutral order here is:

• Η ξεκούραση είναι ουσιαστική για την ανάρρωση.

But you could also hear:

• Για την ανάρρωση, η ξεκούραση είναι ουσιαστική.

That version puts more focus on for recovery.

So Greek allows movement for emphasis, but the original sentence is the most straightforward and natural basic order.

Not exactly.

ουσιαστικός / ουσιαστική / ουσιαστικό literally has the sense of:

• essential
• substantial
• fundamental

In this sentence, ουσιαστική is stronger than just important. It suggests that rest is not merely helpful, but really necessary or central to recovery.

Other possible adjectives would be:

• σημαντική = important
• απαραίτητη = necessary
• βασική = basic / key

So ουσιαστική gives a meaning close to essential.

Yes, absolutely.

That would mean:

• Rest is important for recovery

The difference is mainly nuance:

• ουσιαστική = more like essential, crucial
• σημαντική = more like important

So your alternative is grammatically correct, but slightly weaker in meaning.

Not exactly.

ανάρρωση means recovery, especially the process of getting better after illness, injury, or weakness.

It is different from:

• θεραπεία = treatment / therapy
• ίαση = healing / cure (often more formal)
• θεραπεία ανάρρωσης would be something more specific and not the same basic idea

So in this sentence, ανάρρωση is specifically about recovering, not about the medical treatment itself.