Cuando llegue septiembre, quiero volver a la universidad.

Because after cuando referring to a future event, Spanish normally uses the present subjunctive.

So:

• Cuando llegue septiembre... = When September arrives...
• not Cuando llega septiembre... if you mean a one-time future situation
• not Cuando llegará septiembre..., because Spanish does not usually use the future tense after cuando in this kind of clause

This is a very common pattern:

• Cuando tenga tiempo, te llamo.
• Cuando termine el curso, viajaré.

English uses the present after when too (when September arrives), but Spanish goes one step further and uses the subjunctive when the event is still in the future and not yet realized.

Use cuando + subjunctive for a future or unknown event.

• Cuando llegue septiembre, quiero volver a la universidad.
• When September arrives, I want to go back to university.

Use cuando + indicative for habitual actions, facts, or things that already happen regularly.

• Cuando llega septiembre, las clases empiezan otra vez.
• When September arrives, classes start again.

So the difference is roughly:

• future/not yet happened → subjunctive
• habitual/factual → indicative

Because quiero describes your current desire or intention.

You are saying that right now, you want something to happen later:

• Ahora quiero volver a la universidad cuando llegue septiembre.

This is very natural in both Spanish and English:

• I want to go back to university when September arrives.

You could also use a future form if you want to focus more on what will happen:

• Cuando llegue septiembre, volveré a la universidad. = When September arrives, I’ll go back to university.

But quiero volver emphasizes desire/intention, not just the future action.

Here, volver a means to return to or to go back to.

• volver a la universidad = to go back to university

Be careful: volver a + infinitive can also mean to do something again.

For example:

• Volví a leer el libro. = I read the book again.

So volver a has two common uses:

1. volver a + place → return to a place

   • Volver a casa
   • Volver a Madrid

2. volver a + infinitive → do something again

   • Volver a estudiar
   • Volver a intentarlo

In your sentence, it is clearly the first meaning: returning to a place/institution.

Because after verbs of movement like ir, volver, entrar, Spanish usually uses a before the destination.

So:

• volver a la universidad
• ir a la universidad
• entrar a la universidad (in some varieties; in Spain entrar en is also common depending on meaning)

The article la is also normal here because universidad is a countable noun and refers to the institution/place in a general but specific way.

Compare:

• Voy a la universidad. = I’m going to university / the university.
• Estudio en la universidad. = I study at university / at the university.

In English, university sometimes appears without the, but Spanish often keeps the article.

Yes. Regresar and volver can both mean to return / go back.

• Quiero volver a la universidad.
• Quiero regresar a la universidad.

Both are correct. In everyday speech, volver is often very common and natural.

There can be small style differences:

• volver often feels very ordinary and conversational
• regresar can sound a little more formal in some contexts

But in this sentence, either works well.

In Spanish, months usually do not take an article when used in a basic way.

• Llega septiembre.
• En septiembre...
• Es septiembre.

So Cuando llegue septiembre is perfectly normal.

Sometimes you may see an article with months in other structures, especially when the month is modified or specified more heavily:

• el septiembre de 2020
• el próximo septiembre is less common than el próximo mes de septiembre or simply el próximo septiembre in some usages, but stylistically it depends

For a simple sentence like this, septiembre without an article is the normal choice.

Because in Spanish, names of months are normally written with lowercase, not capitals.

So:

• septiembre
• octubre
• lunes
• enero

This is different from English, where September must be capitalized.

Yes, and it is very natural.

• Cuando llegue septiembre, quiero volver a la universidad.
• En septiembre quiero volver a la universidad.

They are similar, but not identical in nuance:

• Cuando llegue septiembre... focuses on the moment when September arrives
• En septiembre... simply places the action in September

So the first sounds a bit more like once September comes, while the second just means during September.

Because Cuando llegue septiembre is an introductory subordinate clause, and it is very common to separate it with a comma before the main clause:

• Cuando llegue septiembre, quiero volver a la universidad.

This helps readability.

In short sentences, punctuation can sometimes vary, but using the comma here is standard and very natural.

Normally, it refers to the next relevant September or the September the speaker has in mind from context.

• Cuando llegue septiembre usually means when September comes/arrives
• in real life, listeners understand which September is meant from the situation

It does not usually mean every September in this sentence, because the subjunctive points to a future event rather than a habitual one.

If you wanted a habitual meaning, you would be more likely to use the indicative and a different overall context, for example:

• Cuando llega septiembre, siempre pienso en volver a la universidad.

Not if you mean a future event that has not happened yet.

In this sentence, cuando llegue septiembre is the right choice because the arrival of September is still in the future.

If you say cuando llega septiembre, it usually sounds like:

• a repeated or habitual event
• a general truth
• a descriptive statement

So for When September arrives, I want to go back to university, the natural form is:

• Cuando llegue septiembre, quiero volver a la universidad.

Not exactly.

In Spain, universidad means university. A native English speaker might sometimes translate it as college in casual English, but the Spanish word itself is university.

So:

• volver a la universidad = go back to university

In British English, go to university matches very closely. In American English, people may often say go back to college in casual speech, depending on context, even if the Spanish says universidad.

No, not as a complete normal sentence.

Volver is an infinitive, so it needs something that makes it work in the sentence, such as:

• a conjugated verb: quiero volver
• a future form: volveré
• a modal expression: voy a volver, pienso volver

Correct options include:

• Cuando llegue septiembre, quiero volver a la universidad.
• Cuando llegue septiembre, volveré a la universidad.
• Cuando llegue septiembre, voy a volver a la universidad.

So quiero is doing important grammatical work here.