O dentista disse que eu tenho uma cárie pequena, mas fácil de tratar.

Because dentista is one of those nouns that can be either masculine or feminine depending on the person being referred to. The ending -a does not always mean feminine in Portuguese.

• o dentista = the male dentist
• a dentista = the female dentist

So in this sentence, o tells you the dentist is male.

O is the definite article, meaning the.

Portuguese uses articles very naturally before professions or known people when talking about a specific person. Here, o dentista means the dentist, probably the speaker’s dentist or a dentist already understood from context.

Disse is the pretérito perfeito form of the verb dizer = to say / to tell.

It means the dentist said this at a specific moment in the past. It is an irregular form:

• eu disse = I said
• ele/ela disse = he/she said

So O dentista disse... means the action of speaking is completed.

Que introduces a subordinate clause and here means that.

• O dentista disse que... = The dentist said that...

In English, that is often omitted: The dentist said I have...
In Portuguese, que is usually kept.

Not always. Portuguese often drops subject pronouns because the verb ending already shows the person.

• eu tenho = I have
• tenho = also I have

So the sentence could also be:

• O dentista disse que tenho uma cárie pequena...

Including eu can add clarity, emphasis, or simply sound more explicit. In European Portuguese, leaving it out is very common.

Yes, cárie is a feminine noun, so it takes feminine agreement:

• uma cárie
• cárie pequena

That is why you see:

• uma instead of um
• pequena instead of pequeno

In Portuguese, adjectives often come after the noun.

So:

• uma cárie pequena = a small cavity

That is the most neutral, straightforward order.
You can sometimes put adjectives before the noun, but that often sounds more literary, emphatic, or stylistically marked.

Both adjectives agree with cárie, but not all adjectives show gender in the same way.

• pequeno / pequena changes form for masculine and feminine
• fácil has the same singular form for masculine and feminine

So:

• um problema fácil
• uma cárie fácil

In the plural, it does change:

• fáceis

So fácil is feminine here in meaning, but its singular form stays fácil.

Because Portuguese normally uses the pattern:

• adjective + de + infinitive

So:

• fácil de tratar = easy to treat
• difícil de resolver = difficult to solve
• bom de ouvir = good to listen to

This is a very common structure. English speakers often want to remove the de, but in Portuguese it is needed here.

Mas means but and introduces a contrast.

Here the contrast is roughly:

• having a cavity sounds negative,
• but it is small and easy to treat, which is reassuring.

It also links two descriptive ideas about cárie:

• pequena
• fácil de tratar

So the sentence softens the bad news.

In Portuguese, it is standard to put a comma before mas when it joins two contrasting parts of a sentence.

So:

• ..., mas ...

This works much like English punctuation before but.

Yes. Some natural alternatives are:

• O dentista disse que tenho uma cárie pequena, mas fácil de tratar.
• O dentista disse que eu tenho uma pequena cárie, mas fácil de tratar.

The first is very common because Portuguese often drops eu.
The second moves pequena before the noun, which is possible, though uma cárie pequena is the more neutral order.