O doutor disse que eu preciso descansar porque estou com dor nas costas.

O means the, so o doutor refers to a specific doctor, usually one already known in the situation.

In this sentence, the speaker is talking about the doctor who examined them or spoke to them, not just any doctor.

• o doutor = the doctor
• um doutor = a doctor

Portuguese often uses the definite article very naturally in places where English might also use the.

Often, yes, in this context.

• médico is the more literal word for physician / doctor
• doutor is also commonly used in Brazil to refer to a doctor, especially in everyday speech

So both of these can work:

• O doutor disse...
• O médico disse...

In Brazil, doutor can also be used as a respectful title, sometimes even beyond medicine, so médico is more precise, while doutor can sound a little more conversational or socially natural.

Disse is the preterite form of dizer.

• infinitive: dizer = to say / to tell
• preterite: disse = said / told

It refers to a completed action in the past:

• O doutor disse... = The doctor said...

This is an irregular verb, so disse does not look very much like dizer.

Yes. Portuguese dizer often covers both ideas.

In this sentence:

• O doutor disse que eu preciso descansar = The doctor said that I need to rest

But in English, depending on context, you might also translate it as:

• The doctor told me that I need to rest

If Portuguese wants to make the person receiving the message explicit, it can add an indirect object:

• O doutor me disse que... = The doctor told me that...

So disse is flexible, and English chooses said or told depending on context.

Here que means that and introduces the next clause.

• disse que eu preciso descansar
• said that I need to rest

In English, that is often optional:

• The doctor said I need to rest
• The doctor said that I need to rest

In Portuguese, que is usually kept in this structure. Leaving it out would sound unnatural.

Yes, it could.

Portuguese often drops subject pronouns because the verb ending already shows the subject:

• preciso already tells you it means I need

So both are possible:

• O doutor disse que eu preciso descansar
• O doutor disse que preciso descansar

Why include eu then?

• for clarity
• for emphasis
• because after que, the speaker may want to make the subject extra clear

Since the sentence starts with O doutor, adding eu helps avoid any momentary confusion about who needs to rest.

In Brazilian Portuguese, the most common and standard pattern is:

• precisar + infinitive verb

So:

• preciso descansar = I need to rest
• preciso estudar = I need to study

But when precisar is followed by a noun, de is normally used:

• preciso de descanso = I need rest
• preciso de ajuda = I need help

So the basic contrast is:

• precisar + verb → preciso descansar
• precisar de + noun → preciso de descanso

You may occasionally hear precisar de + infinitive in some varieties, but preciso descansar is the normal Brazilian form.

Because here it means because.

Portuguese has several similar-looking forms:

• porque = because
• por que = why / for what reason
• por quê = why, usually at the end of a question
• porquê = the reason, a noun

In this sentence, the second part gives the reason:

• porque estou com dor nas costas
• because I have back pain

So porque is the correct choice.

Because this is a temporary condition, so Portuguese uses estar, not ser.

• estou com dor = I am in pain / I have pain
• sou would be wrong here

Very roughly:

• ser is for identity, characteristics, definitions, origin, time, etc.
• estar is for states, conditions, location, and temporary situations

Pain is a current physical condition, so:

• estou com dor = correct
• sou com dor = incorrect

Literally, estou com dor means I am with pain.

That sounds strange in English, but it is a very normal Portuguese structure. Portuguese often uses:

• estar com + noun

for temporary physical states or symptoms.

Examples:

• estou com febre = I have a fever
• estou com sono = I am sleepy
• estou com dor de cabeça = I have a headache
• estou com dor nas costas = I have back pain

So even though English usually says I have, Portuguese often says I am with in these cases.

Nas is a contraction of:

• em + as = nas

And costas means back as the body part.

So:

• dor nas costas = pain in the back
• literally: pain in the backs / in the back area

A very important detail: costas is normally plural in Portuguese when it means someone's back.

So you say:

• as costas = the back
• dor nas costas = back pain

Not na costa in this meaning.

That is just how Portuguese expresses this body part. Costas is conventionally plural.

So even though English says:

• my back hurts

Portuguese says:

• minhas costas doem
• estou com dor nas costas

This is something you mostly just have to learn as a fixed vocabulary item:

• as costas = back

It is feminine plural, which is why the contraction is nas and not nos.

Yes, it is a very common and natural expression for back pain.

Some related expressions are:

• Estou com dor nas costas. = I have back pain.
• Minhas costas estão doendo. = My back hurts.
• Dor lombar = lower back pain / lumbar pain

For everyday speech, dor nas costas is one of the most common ways to express this idea.