Podczas rehabilitacji robię ćwiczenie wzmacniające.

Because podczas requires the genitive case.

• Dictionary form: rehabilitacja
• Genitive singular: rehabilitacji

So podczas rehabilitacji means during rehabilitation / during rehab. This is a fixed grammar pattern: podczas + genitive.

It is in the accusative singular, because it is the direct object of robię.

However, with a neuter inanimate noun like ćwiczenie, the nominative and accusative forms look the same:

• nominative: ćwiczenie
• accusative: ćwiczenie

The adjective also matches it:

• wzmacniające = neuter singular nominative/accusative

Because it has to agree with ćwiczenie.

Ćwiczenie is:

• singular
• neuter
• accusative here

So the adjective must also be:

• singular
• neuter
• accusative

That gives wzmacniające.

Compare:

• wzmacniający for a masculine noun
• wzmacniająca for a feminine noun
• wzmacniające for a neuter noun

Because robić ćwiczenie / robić ćwiczenia is a normal Polish way to say to do an exercise / to do exercises.

This works much like English do an exercise.

Other possibilities exist:

• wykonuję ćwiczenie wzmacniające — more formal, more like I perform a strengthening exercise
• ćwiczę — I exercise / I practice, but it does not name the exercise in the same way

So robię ćwiczenie is natural and everyday.

Yes, but the sentence would not be exactly the same.

• Ćwiczę podczas rehabilitacji = I exercise during rehab
• Robię ćwiczenie wzmacniające = I do a strengthening exercise

So ćwiczę focuses more on the activity in general, while robię ćwiczenie points to a specific exercise.

The singular means one specific exercise.

If you mean a whole set or general rehab work, Polish often uses the plural:

• Podczas rehabilitacji robię ćwiczenia wzmacniające.

That can sound more natural in many real-life situations, because rehab usually involves more than one exercise.

So:

• singular = one exercise
• plural = exercises in general or a routine

It usually does not show that directly, because Polish has no articles.

So robię ćwiczenie wzmacniające can mean:

• I am doing a strengthening exercise
• I am doing the strengthening exercise

Context tells you which one is meant.

If needed, Polish can make it clearer with other words, but normally it just leaves this to context.

No. Polish word order is fairly flexible.

The original order is neutral:

• Podczas rehabilitacji robię ćwiczenie wzmacniające.

You could also say:

• Robię ćwiczenie wzmacniające podczas rehabilitacji.

The basic meaning stays the same, but the emphasis changes slightly. Polish relies a lot on case endings, so word order is freer than in English.

Robię is:

• 1st person singular
• present tense
• from the imperfective verb robić

Imperfective present in Polish can mean:

• something happening right now
• something done regularly / habitually

So depending on context, robię can mean:

• I am doing
• I do

In this context, rehabilitacja usually means physical rehabilitation or physiotherapy/rehab treatment.

For English speakers, this word can be a little broader than English rehabilitation. In everyday Polish, it is very commonly used for medical rehab after injury, surgery, or pain problems.

So in this sentence, it most likely refers to physical rehab sessions, not something like social or legal rehabilitation.