Lekarz zbada ciało.

Because zbadać is a perfective verb, and zbada is its future tense form: he/she will examine.

In Polish, aspect matters a lot:

• badać = imperfective, to examine / to be examining / to examine repeatedly
• zbadać = perfective, to examine fully / to complete the examination

So:

• Lekarz bada ciało. = The doctor is examining the body / examines the body
• Lekarz zbada ciało. = The doctor will examine the body

With perfective verbs, the present-tense-looking form often has future meaning.

Lekarz is in the nominative case, because it is the subject of the sentence — the person doing the action.

Here, lekarz means:

• doctor (as a dictionary form)
• the doctor or a doctor, depending on context

Its nominative singular form is lekarz.

Ciało is in the accusative case, because it is the direct object — the thing being examined.

The verb zbadać takes a direct object in the accusative:

• zbadać kogo? co? = to examine whom? what?

So:

• ciało = accusative singular

A useful thing to know is that for many neuter nouns, the nominative and accusative forms are the same. So:

• nominative: ciało
• accusative: ciało

That is why it looks unchanged.

Polish has no articles, so there is no direct equivalent of the or a/an.

So Lekarz zbada ciało could mean:

• The doctor will examine the body
• A doctor will examine the body

The exact meaning depends on context.

If Polish speakers want to make something more specific, they usually do it through context, word order, or extra words like:

• ten lekarz = this doctor
• jakieś ciało = some body

Not only. Ciało is a general word meaning body.

Depending on context, it can mean:

• a living human body
• an animal body
• a dead body
• a physical body in a more abstract or scientific sense

In this sentence, because of lekarz and zbada, it most naturally sounds like a medical examination of a body or the body.

Yes. Polish word order is more flexible than English because case endings show grammatical roles.

The neutral order here is:

• Lekarz zbada ciało.

But you could also say:

• Ciało zbada lekarz.
• Zbada ciało lekarz.

These all keep roughly the same basic meaning, but the emphasis changes:

• Lekarz zbada ciało. = neutral
• Ciało zbada lekarz. = emphasis on the body
• Zbada ciało lekarz. = emphasis on the action or a more marked style

English usually cannot do this so freely.

A rough pronunciation is:

• zbada ≈ zba-da

Helpful notes:

• z = like z in zoo
• b = normal b
• a = like a in father
• stress is usually on the second-to-last syllable

So the stress is:

• zBAda

The initial zb cluster may feel unusual for English speakers, but Polish allows consonant groups like that.

A rough guide is:

• ciało ≈ CHA-wo

More precisely:

• ci before a vowel sounds like a soft ć, something like ch in cheer, but softer
• a = a as in father
• ł is pronounced like English w
• stress is on the first syllable here, because it is a two-syllable word: CIA-ło

So:

• ciało sounds roughly like CHA-wo, though the first sound is softer than normal English ch

Both words exist, but they are not always used in exactly the same way.

• lekarz = the standard general word for doctor in the medical sense
• doktor can also mean doctor, but it may sound more like a title, and in some contexts it can also mean someone with a doctoral degree

So in an ordinary medical sentence, lekarz is often the most natural choice.

Yes, zbada is 3rd person singular:

• on / ona / ono zbada = he / she / it will examine

The plural is:

• zbadają = they will examine

So:

• Lekarz zbada ciało. = The doctor will examine the body.
• Lekarze zbadają ciało. = The doctors will examine the body.

No, not this exact sentence.

Lekarz zbada ciało specifically means The doctor will examine the body, because zbada is the perfective future.

If you want is examining, you would normally use the imperfective verb:

• Lekarz bada ciało.

So the aspect difference is important:

• bada = is examining / examines
• zbada = will examine, with the idea of completing the examination