Dummodo puella quieta maneat, medica dicit eam ad scholam paucis diebus redituram esse.

Because dummodo here means provided that / as long as, and it commonly introduces a clause with the subjunctive.

So:

• dummodo puella quieta maneat = provided that the girl remains calm/quiet

The subjunctive is not mainly showing doubt here. It is being used because this kind of subordinate clause regularly takes it after dummodo.

Here dummodo means provided that, so long as, or as long as.

It introduces a condition, but not the ordinary if-type condition you would get with si. Instead, it often means something like:

• on condition that
• provided only that

So the sentence sets a condition first, and then gives the doctor’s statement.

Quieta agrees with puella.

• puella is feminine singular nominative
• so the adjective describing it must also be feminine singular nominative
• therefore: quieta

This is standard adjective agreement in Latin: adjectives match the nouns they describe in gender, number, and case.

It is best understood as predicative with maneat.

So the sense is not mainly the quiet girl remains, but rather:

• the girl remains quiet/calm

That is why quieta works closely with maneat. It describes the state the girl is to stay in.

Dicit means says or is saying. Latin often uses the present here just as English does when reporting what someone says now.

So the basic structure is:

• the doctor says ...

Then what follows is an indirect statement reporting what the doctor says will happen.

Because medica is the feminine nominative singular form, meaning female doctor.

Since medica is the subject of dicit, it has to be in the nominative.

So:

• medica dicit = the female doctor says

If the doctor were male, you would expect medicus dicit.

Because eam is the accusative singular feminine form, and it is the subject of the indirect statement.

In Latin, indirect statements usually use:

• an accusative subject
• plus an infinitive

So in:

• medica dicit eam ... redituram esse

eam is the person who is said to be going to return.

If it were the subject of a normal finite verb, you would expect nominative ea. But with an infinitive construction, Latin uses the accusative instead.

This is the standard Latin indirect statement construction, often called accusative and infinitive.

The parts are:

• eam = the accusative subject, her
• redituram = future active participle, feminine accusative singular
• esse = infinitive to be

Together, redituram esse means to be going to return, and the whole phrase means:

• that she is going to return

So after dicit, Latin does not usually say that with a finite verb. Instead, it uses this accusative + infinitive structure.

Because it agrees with eam.

Since eam is:

• feminine
• singular
• accusative

the future participle must match it:

• redituram

This participle comes from redeo, redire and means about to return / going to return.

So:

• eam redituram esse = that she is going to return

If the subject were masculine, it would be rediturum esse with eum.

Because Latin forms the future infinitive active with:

• the future active participle
• plus esse

So:

• redituram esse = to be going to return

This is the normal way to express future time inside an indirect statement.

Compare:

• eam redire = that she returns / is returning
• eam redituram esse = that she will return / is going to return

So the participle shows the future idea, and esse completes the infinitive construction.

Paucis diebus is ablative plural.

Here it expresses time within which, a very common Latin use of the ablative. It means:

• within a few days
• often more naturally in English: in a few days

So the doctor says that she will return to school within a few days.

Ad with the accusative often shows motion toward a place.

So:

• ad scholam redire = to return to school

Latin often uses ad for going to a place. In some contexts Latin can also use other constructions for place names or certain special nouns, but here ad scholam is perfectly normal and straightforward.

They are viewed from the perspective of dicit.

• maneat is present subjunctive in the condition: provided that she remains calm
• redituram esse is future relative to dicit: that she will return

So the idea is:

• now the doctor says this
• if the girl remains calm
• then she will return to school in a few days

In other words, the calmness is the condition, and the return is the future result.

Latin word order is flexible, and putting the condition first is very natural.

By opening with:

• Dummodo puella quieta maneat

the sentence immediately frames everything that follows as conditional. This gives the condition emphasis:

• Provided that the girl remains calm, ...

That is a very common stylistic choice in Latin.

Not without changing the nuance.

• si would mean a more ordinary if
• dummodo means provided that / as long as, often with the sense this one condition must be met

So dummodo is stronger and more specific here. It sounds like the doctor is saying:

• She can go back in a few days, but only on this condition: she must remain calm.

That special nuance is why dummodo is a good choice.