A settembre torno a studiare in biblioteca.

A settembre is the most natural everyday way to say in September or when September comes in this kind of sentence.

A few useful distinctions:

• a settembre = in September
• da settembre = from September onward / starting in September
• in settembre = possible, but often sounds more formal, literary, or regional

So in normal conversation, A settembre torno... is exactly what you would expect.

Italian often uses the present tense for the future when the time is already clear from the context.

Here, A settembre tells you the action is future, so torno can naturally mean I’m going back / I go back in a future sense.

This is similar to English sentences like:

• In September I start again
• Next week I leave

Italian could also use the future:

• A settembre tornerò a studiare in biblioteca.

That is also correct, but torno sounds very natural and conversational.

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

So:

• torno a studiare = I go back to studying / I start studying again

It suggests that the speaker studied before, then stopped for a while, and is now starting again.

This structure is very common:

• torno a lavorare = I go back to work
• torno a casa = I return home
  
  • but notice: here there is no second verb, so it just means I return

In your sentence, torno a studiare is not just I return; it is I return to the activity of studying.

Because after tornare meaning to go back to doing something, Italian normally uses the pattern:

• tornare a + infinitive

So:

• tornare a studiare
• tornare a leggere
• tornare a parlare

The a is part of the structure. You would not normally say torno studiare.

This is one of those verb patterns that just has to be learned:

• cominciare a + infinitive
• provare a + infinitive
• smettere di + infinitive

Different verbs take different prepositions.

Italian usually leaves out subject pronouns when they are not needed, because the verb ending already shows who the subject is.

Here:

• torno = I return / I go back

The ending -o tells you the subject is I, so io is unnecessary.

You could say:

• Io torno a studiare in biblioteca.

But that would usually add emphasis, contrast, or clarification, for example:

• Io torno a studiare, lui invece continua a lavorare.
  I’m going back to studying, while he keeps working.

So in a neutral sentence, leaving out io is the most natural choice.

In biblioteca is the normal way to say in/at the library when you are talking generally about being there as a place where an activity happens.

So:

• studiare in biblioteca = to study in/at the library

A few comparisons:

• in biblioteca = in/at the library, in a general sense
• nella biblioteca = in the library, often meaning a specific library already identified
• alla biblioteca = less natural here in standard Italian; it may be used in some contexts or regions, but it is not the usual choice in this sentence

So if you are simply saying where you study, in biblioteca is the standard, idiomatic option.

Yes, Italian word order is fairly flexible, but the version you have is very natural.

• A settembre torno a studiare in biblioteca.

Starting with A settembre puts the time frame first, which is very common.

Other orders are possible, for example:

• Torno a studiare in biblioteca a settembre.

This is grammatical, but it may sound a little less smooth in many contexts.

You can also shift the focus slightly:

• A settembre torno in biblioteca a studiare.

That version can make it sound a bit more like I’m going back to the library to study, with more emphasis on returning to the place.

So yes, the order can change, but the original sentence is probably the most neutral and natural phrasing.

Usually a settembre means in September / when September comes, not necessarily one exact day.

So it can refer to:

• something happening at some point in September
• a plan associated with that month
• a return to a routine when September begins

If you wanted to be more precise, you could add a date or other detail:

• Il 5 settembre torno a studiare in biblioteca.
• Da settembre torno a studiare in biblioteca.

The second one, da settembre, makes the idea of starting in September and continuing after that clearer.