La profesora me recomendó una libreta nueva porque la mía ya está casi llena.

Because here we are talking about a specific teacher: the teacher. In Spanish, it is very common to use the definite article (el, la, los, las) with nouns when referring to a specific person or thing.

So:

• La profesora = the teacher
• Una profesora = a teacher

A useful contrast:

• Es profesora = She is a teacher
  Here, after ser, professions usually appear without an article.
• La profesora me recomendó... = The teacher recommended to me...
  Here, la profesora is the subject, referring to a particular teacher.

Me is an indirect object pronoun meaning to me.

In me recomendó una libreta nueva:

• recomendó = recommended
• me = to me
• una libreta nueva = a new notebook

So literally, the structure is:

• La profesora me recomendó una libreta nueva
  = The teacher recommended a new notebook to me

Spanish often uses these small object pronouns where English may use a prepositional phrase like to me.

Other examples:

• Me dio un libro = She gave me a book
• Te explicó la lección = She explained the lesson to you
• Nos compró comida = She bought us food

Recomendó is the third-person singular preterite form of recomendar.

The accent mark is needed to show both:

1. the correct pronunciation, and
2. that it is past tense, not present tense.

Compare:

• recomiendo = I recommend
• recomienda = she/he recommends
• recomendó = she/he recommended

Without the accent, recomendo would be incorrect.

This is a very common pattern in the preterite for -ar verbs:

• habló = she/he spoke
• compró = she/he bought
• estudió = she/he studied
• recomendó = she/he recommended

The preterite (recomendó) is used because the sentence refers to a completed action: the teacher made a recommendation at a particular moment.

So it presents the recommendation as a finished event.

• La profesora me recomendó... = The teacher recommended...

If you used the imperfect, recomendaba, it would usually suggest something ongoing, habitual, or backgrounded, for example:

• La profesora siempre me recomendaba libros
  = The teacher used to recommend books to me

So here, recomendó is the natural choice because it sounds like a specific recommendation.

Both are possible, but they can sound slightly different.

una libreta nueva

This is the most straightforward way to say a new notebook meaning a notebook that is unused or new in condition.

una nueva libreta

This can also mean a new notebook, but it often carries more of the sense of another/new replacement notebook.

In this sentence, the reason is that the old one is almost full, so the teacher recommends a replacement. Because of that context, una libreta nueva sounds very natural.

A helpful idea:

• adjective after the noun = often more descriptive/literal
• adjective before the noun = sometimes more subjective or can slightly change nuance

Compare:

• un coche nuevo = a brand-new car
• un nuevo coche = a new car / another car

They are similar, but not always identical in usage.

In Spain, libreta often means a notebook, especially a smaller or ordinary writing notebook. Cuaderno also means notebook/exercise book and is very common too.

Depending on context:

• libreta can suggest a notebook for notes
• cuaderno can be a school notebook or exercise book

In many situations, both would be understood. Here, libreta simply refers to a notebook.

La mía is a possessive pronoun, meaning mine.

It replaces the noun libreta, so instead of repeating it, Spanish says:

• la mía = mine / my one

In the sentence:

• porque la mía ya está casi llena
• literally: because mine is already almost full

This avoids repeating libreta:

• ...porque mi libreta ya está casi llena is also correct,
• but la mía sounds more natural when the noun has just been mentioned.

Notice that la is used because libreta is feminine singular.

Compare:

• el mío = mine (masculine singular)
• la mía = mine (feminine singular)
• los míos = mine (masculine plural)
• las mías = mine (feminine plural)

Mía has an accent mark to show the stress and pronunciation.

It is pronounced with two syllables:

• mí-a

The accent helps show that the i and a are pronounced separately, not merged into a single syllable.

This happens with possessive pronouns and adjectives like:

• mío / mía
• tuyo / tuya
• suyo / suya

So:

• la mía = mine
• el mío = mine

Because llena describes a state or condition, not an essential characteristic.

In Spanish:

• ser is used for identity, defining characteristics, origin, etc.
• estar is used for states, conditions, or results

A notebook being full is a condition, so:

• está casi llena = is almost full

You would not normally say es casi llena here.

Compare:

• Es grande = It is big
• Está abierta = It is open
• Está rota = It is broken
• Está llena = It is full

Because llena agrees with libreta, which is feminine singular.

In Spanish, adjectives must agree in gender and number with the noun they describe.

• la libreta → feminine singular
• therefore llena → feminine singular

Examples:

• El cuaderno está lleno = The notebook is full
• La libreta está llena = The notebook is full
• Los cuadernos están llenos = The notebooks are full
• Las libretas están llenas = The notebooks are full

Even though libreta is not repeated in la mía, it is still understood, so the adjective remains feminine singular: llena.

Ya here means already.

So:

• ya está casi llena = is already almost full

It adds the idea that the notebook has reached that point sooner than expected, or that this is now the situation.

Without ya:

• está casi llena = it is almost full

With ya:

• ya está casi llena = it is already almost full

It is a small word, but very common and very useful in Spanish.

Because porque is the conjunction meaning because.

In Spanish:

• porque = because
• por qué = why
• porqué = the reason / the motive
• por que = a less common combination in specific structures

In your sentence, the second part explains the reason:

• porque la mía ya está casi llena
• because mine is already almost full

So porque is the correct form.

Yes. Spanish often omits subject pronouns and even subject nouns when the context is clear.

So if it is already obvious who is being talked about, you could say:

• Me recomendó una libreta nueva porque la mía ya está casi llena.

The verb form recomendó already tells you the subject is he/she/it. If the listener already knows it was the teacher, the sentence is perfectly natural.

Including La profesora is useful when:

• you are introducing who did the action,
• you want to be clear,
• or you want to emphasize that it was the teacher.

The order is very typical Spanish:

• La profesora = subject
• me = indirect object pronoun
• recomendó = verb
• una libreta nueva = direct object

So the pattern is:

Subject + indirect object pronoun + verb + direct object

That is very common in Spanish:

• Mi madre me compró un libro
• El profesor nos explicó la gramática
• Tu amiga te mandó un mensaje

English often puts to me after the object, but Spanish usually places the indirect object pronoun before the conjugated verb.

Yes:

• La profesora me recomendó una libreta nueva porque mi libreta ya está casi llena.

That is grammatically correct. However, it repeats libreta, so it sounds a bit less elegant.

Using la mía is more natural because Spanish often avoids repeating a noun that is already clear from context.

So:

• porque mi libreta ya está casi llena = correct
• porque la mía ya está casi llena = also correct, and usually more natural here