Ella me dijo que quería volver a la biblioteca mañana.

Not usually. In Spanish, the verb often already tells you who the subject is, so me dijo que quería volver a la biblioteca mañana would normally be enough.

Adding ella can:

• give emphasis
• create contrast, as in she told me, not someone else
• make the subject clearer from context

So ella is grammatical, but often optional.

Me is an unstressed object pronoun meaning to me here.

In Spanish, unstressed object pronouns normally go before a conjugated verb:

• me dijo
• te dijo
• le dijo

So me dijo is the normal word order.
Dijo me is not standard modern Spanish.

It is the indirect object pronoun. It tells you who received the information.

So:

• Ella dijo... = she said...
• Ella me dijo... = she told me...

Spanish often uses these little pronouns where English might use a separate phrase like to me.

Dijo is the preterite of decir. It refers to a completed action in the past: one finished act of telling.

That fits this sentence well, because the telling is presented as a single completed event.

By contrast, decía is the imperfect, which would usually suggest:

• an ongoing action in the past
• repeated telling
• background description

So:

• me dijo = she told me
• me decía = she was telling me / she used to tell me

Also, decir is irregular in the preterite:

• dije, dijiste, dijo, dijimos, dijisteis, dijeron

Que introduces the reported clause, like that in English.

So:

• Ella me dijo que quería volver...

In English, that is often optional:

• She told me that...
• She told me...

But in Spanish, que is normally required in this kind of sentence.
Leaving it out would sound wrong.

Quería is the imperfect of querer, and here it expresses what she wanted or intended at that time.

This is very natural in reported speech after a past verb like dijo.

Quiso is the preterite, and it often sounds more like:

• she wanted to and made a specific attempt
• she decided to
• a completed moment of wanting

So the nuance changes:

• quería volver = she wanted / was wanting / intended to return
• quiso volver = she wanted to return, often with a more definite or bounded sense, sometimes close to tried to return

In this sentence, quería is the more neutral and natural choice.

Because the sentence is reporting what her wish or intention was at the time she spoke, not necessarily what it is now.

So:

• dijo que quería = she said that she wanted...
• dijo que quiere = she said that she wants...

The second version can be possible in some contexts if the desire is still true now, but quería is the more standard and expected choice in ordinary reported speech with a past reporting verb.

Here volver means to return or to go back.

So volver a la biblioteca means returning to the library.

Be careful: Spanish also has the pattern volver a + infinitive, which means to do something again:

• volver a estudiar = study again

But that is not what is happening here.

In this sentence:

• volver = return
• a la biblioteca = to the library

The a belongs with the destination, not with another verb.

Because biblioteca is a feminine noun:

• la biblioteca

The preposition a combines with:

• el → al

But it does not combine with la.

So:

• a + el parque → al parque
• a + la biblioteca → a la biblioteca

That is why a la is correct here.

No. Biblioteca means library.

This is a very common false friend for English speakers.

If you want bookshop or bookstore, the word is usually librería.

So:

• biblioteca = library
• librería = bookshop / bookstore

Mañana is flexible, but its position can slightly affect emphasis.

The sentence you have is very natural:

• Ella me dijo que quería volver a la biblioteca mañana.

You could also hear:

• Ella me dijo que mañana quería volver a la biblioteca.

Both are grammatical. The second places a little more focus on tomorrow.

In general, Spanish allows time expressions like mañana to move around more than English does, as long as the meaning stays clear.

It can be a little ambiguous without context.

Depending on when the sentence is said, mañana may refer to:

• the day after the original statement
• the day after the current moment of speaking

If the speaker wants to be very clear about the next day relative to the past event, Spanish can use al día siguiente.

So a clearer alternative in some contexts would be:

• Ella me dijo que quería volver a la biblioteca al día siguiente.

That avoids ambiguity.

Yes, absolutely. In fact, that would often sound more natural in everyday Spanish.

Spanish frequently drops subject pronouns when they are not needed.
So both are correct:

• Ella me dijo que quería volver a la biblioteca mañana.
• Me dijo que quería volver a la biblioteca mañana.

The version with ella simply gives extra emphasis or clarity.