Si no hace la siesta, la pediatra dice que luego estará más cansada por la tarde.

Because si without an accent means if.

• si = if
• sí = yes

So in this sentence, Si no hace la siesta... means If she doesn’t take a nap...

The accent changes the meaning completely.

Because hacer la siesta is the normal Spanish expression for to have/take a nap or to take a siesta.

Literally, hacer usually means to do/make, but in fixed expressions it often works differently. Here, you should learn hacer la siesta as a set phrase.

Examples:

• Hace la siesta todos los días. = She takes a nap every day.
• No hace la siesta hoy. = She isn’t taking a nap today.

Even though the literal wording may feel odd to an English speaker, it is completely natural in Spanish.

Because the idiom normally includes the definite article: hacer la siesta.

Spanish often uses articles in places where English does not. You usually do not say just hacer siesta in standard Spanish from Spain.

So:

• hacer la siesta = correct, natural
• hacer siesta = generally not the standard form in Spain

This is something best learned as part of the whole expression.

Pediatra can refer to either a male or female pediatrician. The article tells you the gender of the person:

• el pediatra = the male pediatrician
• la pediatra = the female pediatrician

So la pediatra dice... means the pediatrician is a woman.

This is common with some profession words ending in -a that can still be masculine or feminine depending on the article.

Because it agrees with the person who will be tired. The sentence is talking about a female child or baby, so the adjective is feminine singular:

• cansada = tired, feminine singular
• cansado = tired, masculine singular

So if the child were a boy, you would say:

• Si no hace la siesta... estará más cansado...

Adjectives in Spanish usually agree in gender and number with the person or thing they describe.

Because Spanish often leaves out subject pronouns when the verb form already makes the meaning clear from context.

In hace, the subject could be he/she/it/you (formal), but here the rest of the sentence and the adjective cansada show that it refers to a girl.

A more explicit version could be:

• Si ella no hace la siesta...

But Spanish usually prefers to omit ella unless it is needed for emphasis or clarity.

Because in Spanish, for real or likely future conditions, the pattern is:

• si + present indicative
• main clause with future, present, or another appropriate tense

So:

• Si no hace la siesta, ... estará más cansada.

This is the normal structure for If she doesn’t take a nap, she’ll be more tired later.

English also uses the present after if:

• If she doesn’t nap, she’ll be more tired.

Spanish does not normally use the future after si in this kind of sentence. So Si no hará la siesta... would be wrong here.

Estará is the future tense of estar: she will be.

The pediatrician is talking about the result that will happen later, so the future makes sense:

• estará más cansada = she will be more tired

You could also hear:

• va a estar más cansada

That also means she is going to be more tired and is very common in everyday speech.

Because más means more. The idea is not simply that she will be tired, but that she will be more tired than she would otherwise be.

So:

• cansada = tired
• más cansada = more tired

This comparison is often implied against the alternative situation where she does take the nap.

Here luego means later or then.

It helps show sequence:

• first: she does not take the nap
• later: she will be more tired

So luego estará más cansada means something like:

• then she’ll be more tired
• she’ll be more tired later

In many contexts, luego and después can be quite similar.

They do overlap a bit, but they are not exactly the same.

• luego = later / then
• por la tarde = in the afternoon

So luego gives the sequence, while por la tarde gives the time period more specifically.

Together, they mean something like:

• then she’ll be more tired later in the afternoon
• she’ll be more tired afterwards, in the afternoon

It may sound slightly repetitive if translated word for word into English, but it is natural in Spanish.

In Spain, por la tarde is the usual way to say in the afternoon.

Common time expressions in Spain are:

• por la mañana = in the morning
• por la tarde = in the afternoon
• por la noche = at night / in the evening

In some parts of Latin America, en la tarde is also used, but por la tarde is the standard choice for Spain, which fits your sentence.

Dice que means says that.

It introduces reported speech or reported information:

• La pediatra dice que... = The pediatrician says that...

After decir que, Spanish often keeps the statement structure quite straightforward:

• La pediatra dice que luego estará más cansada...

So the sentence is reporting what the pediatrician says will happen.

Not in this version.

Why not?

• After si in a real, possible condition, Spanish uses the indicative, not the subjunctive: si no hace
• After dice que, you also use the indicative because it is reporting a statement: dice que... estará

So everything here is indicative:

• hace
• dice
• estará

You would need a different structure to trigger the subjunctive.

Yes, in many contexts it could.

For example:

• Si no hace la siesta, la pediatra dice que después estará más cansada por la tarde.

That would still be understandable and natural.
However, luego is very common in Spain and works very well here.

So this is mostly a vocabulary choice, not a grammar issue.

Yes, the future tense in Spanish can sometimes express probability, such as:

• Estará en casa. = She is probably at home.

But in this sentence, that is not the most natural interpretation. Here, estará is a normal future: she will be.

The context makes that clear:

• condition: If she doesn’t take a nap
• consequence: she will be more tired later

So this is about a future result, not speculation.

Yes, in this sentence that is the best natural translation.

Although siesta is also a cultural word in English, here it just refers to a child’s nap. So for understanding the grammar, it is easiest to think:

• hacer la siesta = to take a nap

That will usually sound more natural than keeping the Spanish word in English.