Mój brat jest dentystą, a jego żona jest dentystką.

Because after jest in this kind of sentence, Polish normally uses the instrumental case for professions, roles, and identities.

So:

• dentysta → dictionary form, nominative
• dentystą → instrumental
• dentystka → dictionary form, nominative
• dentystką → instrumental

That is why you get:

• Mój brat jest dentystą
• Jego żona jest dentystką

Meanwhile, the subjects brat and żona stay in the nominative because they are the ones doing the being.

The main cases here are:

• mój brat — nominative, because brat is the subject
• jego żona — nominative, because żona is the subject
• dentystą — instrumental, because it comes after jest
• dentystką — instrumental, for the same reason

So the basic pattern is:

• subject in nominative
• profession/identity after jest in instrumental

Because mój has to agree with brat, and brat is a masculine singular noun.

So:

• mój brat = my brother
• moja siostra = my sister

The possessive mój / moja / moje changes to match the gender and number of the noun it describes.

Because in Polish, a noun ending in -a is often feminine, but not always. Some masculine nouns also end in -a.

Dentysta is one of those masculine nouns. Other examples include:

• mężczyzna = man
• kolega = male friend / colleague

So even though dentysta ends in -a, it is still grammatically masculine, which is why you say:

• mój brat jest dentystą

Its feminine counterpart is:

• dentystka

As a possessive word, jego means his, and it usually does not change form to match the noun after it.

So you get:

• jego brat = his brother
• jego żona = his wife
• jego dzieci = his children

That is different from mój, which does change:

• mój brat
• moja żona
• moje dziecko

So in this sentence:

• mój changes because it is one of the agreeing possessives
• jego stays the same

Often yes, but not exactly.

A commonly links two ideas with a slight contrast or comparison. In many sentences, English translates it as and, but it can also feel like:

• and
• while
• whereas

So here:

• Mój brat jest dentystą, a jego żona jest dentystką

means something like:

• My brother is a dentist, and his wife is a dentist too
• or My brother is a dentist, while his wife is a female dentist

It is not as strong as ale, which means but.

Yes. That is a very common and natural alternative.

There is an important difference:

• with jest, Polish usually uses the instrumental:
  • jest dentystą
  • jest dentystką
• with to, Polish usually uses the nominative:
  • to dentysta
  • to dentystka

So both are possible:

• Mój brat jest dentystą, a jego żona jest dentystką.
• Mój brat to dentysta, a jego żona to dentystka.

For a learner, it is useful to remember:

• jest → often followed by instrumental
• to → often followed by nominative

Because Polish does not have articles like English does.

So:

• brat can mean a brother, the brother, or just brother, depending on context
• dentystą can mean a dentist or the dentist, depending on context

English forces you to choose an article, but Polish usually does not.

In a full standard sentence, repeating jest is completely normal and usually the safest choice:

• Mój brat jest dentystą, a jego żona jest dentystką.

You may sometimes see or hear ellipsis, especially in more compressed style:

• Mój brat jest dentystą, a jego żona dentystką.

But for learners, it is better to keep jest in both parts unless you are already comfortable with omission patterns.

Because swój refers to the possessor being the subject of the same clause.

In the clause:

• jego żona jest dentystką

the subject is żona. So swoja żona would incorrectly suggest that the wife is somehow the possessor of wife, which does not make sense here.

You use jego because the wife belongs to him — the brother mentioned earlier.

A helpful comparison:

• Mój brat lubi swoją żonę = My brother likes his own wife.

Here swój works because the subject is mój brat, and he is also the possessor.

Yes. Polish often marks profession nouns for gender.

Here:

• dentysta = male dentist
• dentystka = female dentist

So the sentence explicitly says:

• the brother is male and is a dentist
• the wife is female and is a dentist

This kind of gender marking is much more visible in Polish than in English.

Yes, it is a bit tricky at first.

A rough guide:

• den-tys-tkom is a useful English-style approximation

A few points:

• y is not exactly English ee or i; it is a short Polish vowel
• stk is a real consonant cluster, so say it clearly: tys-tk
• final ą is a nasal vowel; before or at the end of a word, learners often approximate it as om

So:

• dentystą ≈ den-TIS-tom
• dentystką ≈ den-TIS-tkom

Not perfect, but good enough as a starting point.

Yes, Polish word order is more flexible than English, but the given version is the most neutral and natural for a beginner:

• Mój brat jest dentystą, a jego żona jest dentystką.

You can change word order for emphasis, but that changes the feel of the sentence. For example, moving words around may sound more contrastive or literary.

So the best thing to learn first is the standard pattern:

• possessive + subject + jest + profession
• a
• possessive + subject + jest + profession