Lekarz zbada mózg.

Because zbada comes from the perfective verb zbadać. In Polish, perfective verbs do not normally have a true present-tense meaning. Their present-looking forms are used for the future.

So:

• zbada = will examine / will check
• not is examining

If you want a present meaning, you would usually use the imperfective verb badać:

• Lekarz bada mózg. = The doctor is examining / examines the brain.

This is a question of aspect, which is very important in Polish.

• badać = imperfective
  Focuses on the process, duration, repetition, or ongoing action.
• zbadać = perfective
  Focuses on the action as a completed whole, often with a result.

So Lekarz zbada mózg suggests that the doctor will carry out the examination to completion.

A rough contrast:

• Lekarz bada mózg. = The doctor is examining the brain.
• Lekarz zbada mózg. = The doctor will examine the brain / will complete an examination of the brain.

Lekarz is a noun in the:

• nominative case
• singular
• masculine

It is the subject of the sentence, so nominative is exactly what you would expect.

Its basic dictionary form is also lekarz.

Mózg is the direct object, so it is in the accusative case. However, for many masculine inanimate nouns in Polish, the accusative singular looks exactly the same as the nominative singular.

So:

• nominative: mózg
• accusative: mózg

That is normal.

Compare that with a masculine animate noun, where the accusative usually changes:

• pies = dog (nominative)
• widzę psa = I see the dog (accusative)

But with mózg, no visible change appears.

Polish does not have articles like English a/an and the.

So Lekarz zbada mózg can mean things like:

• The doctor will examine the brain
• A doctor will examine the brain

Which one is intended depends on the context.

This is very normal in Polish, and learners have to get used to understanding definiteness from the situation rather than from an article.

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

The neutral order here is:

• Lekarz zbada mózg.

This is the most straightforward subject–verb–object order.

But other orders are possible for emphasis or focus, for example:

• Mózg zbada lekarz.

This still means roughly the same thing, but it sounds more marked, as if emphasizing the brain or contrasting who will examine it.

For learners, the safest default is the neutral order:

• subject + verb + object

Yes. Lekarz normally means a medical doctor / physician.

It does not usually mean a person with a doctoral degree in the academic sense. For that, Polish uses other wording, depending on context, such as doktor.

So in this sentence, lekarz clearly means a medical professional.

Grammatically, lekarz is a masculine noun. In actual use, it can refer to:

• a male doctor
• sometimes a doctor in a more general or unspecified sense

If you want to make it clearly female, Polish often uses:

• lekarka = female doctor

You may also hear:

• pani lekarz

In this sentence by itself, many people would naturally picture a male or generic doctor, but context matters.

All of those can be possible depending on context.

In medical Polish, zbadać often means:

• examine
• check
• assess
• sometimes test

It usually sounds more clinical and purposeful than just look at. It suggests a real examination meant to produce information or a diagnosis.

So in this sentence, zbada mózg means the doctor will perform an examination of the brain, not just glance at it.

A rough English-friendly pronunciation is:

• LEH-kash ZBA-da moosk

A few useful details:

• lekarz: the final rz sounds like sh here because of final devoicing
• zbada: starts with a real zb- cluster, so pronounce both sounds
• ó in mózg is pronounced like u
• mózg is pronounced roughly like moosk

So the whole sentence sounds approximately like:

• LEH-kash ZBA-da moosk

That is only an approximation, but it is a good start.