Da quando mia figlia è tornata a studiare a settembre, prepara da sola lo zaino ogni sera.

Da quando means since or more literally from when. It introduces the starting point of a situation that continues afterward.

• Da quando mia figlia è tornata a studiare... = Since my daughter went back to studying...
• Quando on its own usually just means when, without the idea of something starting at that moment and continuing.

So da quando is important because the sentence is talking about a change that began then and is still true now.

In Italian, singular family members normally take the possessive without the article:

• mia figlia
• mio padre
• tua sorella

So mia figlia is the normal form.

You may see the article with:

• plurals: le mie figlie
• modified forms: la mia figlia maggiore
• affectionate or stylistically marked uses in some contexts

But in a simple sentence like this, mia figlia is exactly what you would expect.

The verb tornare uses essere as its auxiliary in compound tenses, not avere.

So:

• è tornata = she returned / went back
• not ha tornato

This is common with many intransitive verbs of movement or change of state, such as:

• andare → è andata
• arrivare → è arrivata
• partire → è partita

Because with verbs that take essere, the past participle agrees with the subject in gender and number.

The subject is mia figlia, which is feminine singular, so:

• è tornata = feminine singular

Compare:

• mio figlio è tornato
• le mie figlie sono tornate

Tornare a + infinitive means to go back to doing something or to return to doing something.

So:

• è tornata a studiare = she went back to studying

This structure is very common:

• tornare a lavorare = to go back to work
• tornare a parlare = to speak again
• tornare a vivere = to start living again / go back to living

The a is required in this pattern.

In Italian, months are commonly used with a when meaning in a certain month:

• a settembre = in September
• a maggio = in May

This is a normal time expression. Italian often uses a where English uses in.

Because the past event is only the starting point. The main idea is about a habit that is true now.

Structure:

• Da quando... = since...
• prepara... ogni sera = she prepares... every evening

So the sentence means that from that moment onward, this has been her routine. Italian often uses the present tense for something that started in the past and still continues now, especially with expressions like da or da quando.

Italian usually leaves out subject pronouns when they are not needed, because the verb ending already shows the person and number.

• prepara already means she prepares or he prepares, depending on context
• here the context makes it clear that the subject is mia figlia

So lei prepara is possible, but unnecessary unless you want emphasis or contrast.

Da sola means by herself / on her own.

It emphasizes that she does it independently, without help.

• prepara da sola lo zaino = she packs her schoolbag by herself

Just sola usually means alone in a more general descriptive sense.
In this sentence, da sola is the natural choice because the focus is on doing the action independently.

The definite article lo is used before words beginning with:

• z
• s + consonant
• gn
• ps
• x
• y

Since zaino begins with z, it takes lo:

• lo zaino

Compare:

• il libro
• lo studente
• lo gnomo

Italian often uses the definite article where English prefers a possessive, especially when the owner is already clear from context.

So:

• prepara lo zaino literally = prepares the backpack
• but naturally understood as she packs her backpack

Because the subject is mia figlia, it is obvious whose backpack it is. Italian does this very often with body parts, clothing, and personal belongings.

Yes, both are possible.

• ogni sera = every evening
• tutte le sere = every evening / every night

They are very similar here.
Ogni sera sounds a little more compact and general, while tutte le sere can feel slightly more emphatic, but in most everyday contexts they are interchangeable.

The comma separates the introductory time clause from the main clause:

• Da quando mia figlia è tornata a studiare a settembre,
• prepara da sola lo zaino ogni sera.

It helps the sentence read more clearly. In writing, this is very common when a longer subordinate clause comes first.