Plan rehabilitacji jest gotowy.

Because rehabilitacji is in the genitive case.

In Polish, when one noun modifies another in the sense of X of Y, the second noun is often put in the genitive:

• plan rehabilitacji = plan of rehabilitation / rehabilitation plan
• koniec filmu = end of the film
• adres szkoły = the school’s address

The basic dictionary form is rehabilitacja, but after plan, it changes to rehabilitacji.

So:

• rehabilitacja = nominative, basic form
• rehabilitacji = genitive singular

Plan is in the nominative case because it is the subject of the sentence.

The sentence is built like this:

• Plan rehabilitacji = the rehabilitation plan
• jest gotowy = is ready

So plan is the thing being described as ready.

Because gotowy has to agree with plan.

In Polish, adjectives agree with the noun in:

• gender
• number
• case

Plan is:

• masculine
• singular
• nominative

So the adjective must also be masculine singular nominative:

• gotowy = masculine singular
• gotowa = feminine singular
• gotowe = neuter singular or non-masculine-personal plural

Compare:

• Plan jest gotowy.
• Lista jest gotowa.
• Ćwiczenie jest gotowe.

Jest means is. It is the 3rd person singular present tense of być = to be.

So:

• plan = plan
• jest = is
• gotowy = ready

Together: The rehabilitation plan is ready.

Unlike in Russian and some other Slavic languages, Polish normally does use the present-tense form of to be in sentences like this.

In a normal full sentence, no—you should keep it.

• Plan rehabilitacji jest gotowy. = standard
• Plan rehabilitacji gotowy. = sounds incomplete, elliptical, or like a note/headline

In casual speech, people sometimes shorten things, but for learners, it is best to treat jest as necessary here.

Both are possible, but they are not exactly the same.

• plan rehabilitacji = rehabilitation plan / plan of rehabilitation
• plan na rehabilitację = plan for rehabilitation

The first sounds like a named or established thing: a rehabilitation plan. The second emphasizes the purpose a bit more: a plan intended for rehabilitation.

In many contexts, plan rehabilitacji is the more natural phrase for rehabilitation plan.

Because Polish does not have articles like a/an/the.

So plan can mean:

• a plan
• the plan

The exact meaning depends on context.

That means Plan rehabilitacji jest gotowy could mean:

• A rehabilitation plan is ready
• The rehabilitation plan is ready

In real use, the situation usually makes it clear which one is meant.

Yes, Polish word order is more flexible than English, although the version you have is the most neutral and natural.

Neutral order:

• Plan rehabilitacji jest gotowy.

Other possible orders can shift emphasis:

• Gotowy jest plan rehabilitacji.
  This can sound more literary or emphasize ready.

• Jest gotowy plan rehabilitacji.
  This can sound like There is a rehabilitation plan ready or emphasize the existence of a ready plan.

As a learner, the safest default is:

• subject + jest + adjective

Mostly yes, but like ready in English, it can cover a few related ideas depending on context:

• prepared
• completed enough to be used
• finished and available
• all set

In this sentence, gotowy most likely means something like:

• ready
• prepared
• finished

So the sense is that the rehabilitation plan has been prepared and can now be used.

A rough English-friendly guide is:

re-ha-bee-lee-TA-tsyi

A few useful points:

• ch is not there; the h is pronounced
• ta is stressed because Polish almost always stresses the second-to-last syllable
• cj in -cji sounds roughly like tsy + y/i

More roughly:

• rehabilitacja → re-ha-bee-lee-TA-tsya
• rehabilitacji → re-ha-bee-lee-TA-tsyi

Here it is singular genitive.

The base noun is rehabilitacja = rehabilitation.

Its genitive singular form is rehabilitacji.

This form can look similar to some plural forms in Polish, so it is a very common learner question. In this sentence, though, it clearly means of rehabilitation, singular.

Yes, you could. That means something like The rehabilitation plan is ready too, but the structure is different.

• plan rehabilitacji = literally plan of rehabilitation
• plan rehabilitacyjny = rehabilitation plan with an adjective

Both are possible. Which one sounds more natural can depend on context and style. In medical or administrative language, both types of phrasing are common.

So:

• Plan rehabilitacji jest gotowy.
• Plan rehabilitacyjny jest gotowy.

Both are grammatical, but they are built differently.