La médica dice que debo guardar reposo en casa.

Because the sentence is referring to a female doctor.

• el médico = the male doctor / the doctor
• la médica = the female doctor

In modern Spanish, especially in Spain, médica is a normal and standard feminine form. The article la and the ending -a both show that the doctor is a woman.

Here, la médica means the doctor, referring to a specific person.

Spanish often uses the definite article with professions when talking about a particular person:

• La médica dice... = The doctor says...
• El profesor llega tarde. = The teacher arrives late.

But after ser, professions often appear without an article:

• Es médica. = She is a doctor.

So the article is there because la médica is the subject of the sentence, not because professions always need an article.

Because que introduces the reported statement:

• dice que debo guardar reposo = says that I must rest

In English, that is often optional:

• The doctor says that I must rest
• The doctor says I must rest

In Spanish, que is normally required in this structure. Leaving it out would sound wrong.

Debo is the first person singular form of deber, so it means I must / I should.

That means the person who must rest is I.

Spanish often leaves out subject pronouns when the verb ending already makes the subject clear:

• debo = yo debo

So even though yo is not written, it is understood.

It can mean either, depending on context.

In this sentence, it suggests a fairly strong obligation, so English might translate it as:

• I must
• I should
• I need to

Because this is medical advice, debo sounds stronger than a casual suggestion. It is stronger than debería, which would mean I should in a softer, less direct way.

Both are possible, and both can express obligation.

• debo guardar reposo = I must / should rest
• tengo que guardar reposo = I have to rest

The difference is often one of tone:

• deber can sound a bit more formal, neutral, or advice-like
• tener que can sound a bit more everyday and direct

In a medical sentence, debo sounds very natural.

It is a common medical expression meaning to rest, to stay off your feet, or to observe rest.

Literally, guardar usually means to keep or to put away, and reposo means rest. But together, guardar reposo is an idiomatic phrase. You should understand it as a fixed expression, not word by word.

In context, it means something like:

• to rest
• to take it easy
• to remain resting at home
• to follow the doctor’s order to rest

Not always exactly, but it can come close.

Guardar reposo usually means resting because of illness or medical advice. Depending on the situation, that might mean:

• avoiding work
• staying at home
• lying down more
• possibly staying in bed

If Spanish wants to be more specific about bed rest, it might say:

• guardar cama = to stay in bed
• reposo absoluto = complete rest / strict bed rest

So guardar reposo is a bit broader than just stay in bed.

Because en casa is the normal expression for at home.

• en casa = at home
• en la casa = in the house

So en casa focuses on the idea of home as the place where you live, while en la casa sounds more like the physical building.

That is why:

• Estoy en casa = I am at home
• Estoy en la casa = I am in the house

In this sentence, the meaning is clearly at home, so en casa is the natural choice.

Because a casa expresses movement toward home, while en casa expresses location.

• Voy a casa = I’m going home
• Estoy en casa = I’m at home

In the sentence, the idea is not going somewhere, but resting at home, so en casa is correct.

Yes, absolutely. That would also be correct and natural.

The main difference is style:

• debo sounds a little more formal or measured
• tengo que sounds a little more conversational

Both work well, and both would be understood easily.

In most of Spain, dice is pronounced approximately DEE-theh.

That is because c before e or i is usually pronounced like the th in think in most Peninsular Spanish.

So:

• dice → approximately DEE-theh

In Latin America and some parts of Spain, it is pronounced with an s sound instead:

• approximately DEE-seh

Also, médica has the stress on the first syllable because of the accent mark:

• MÉ-di-ca