En cuanto salga de la facultad, pasaré por la papelería para hacer una fotocopia.

Because after en cuanto when you are talking about a future event that has not happened yet, Spanish normally uses the present subjunctive.

So:

• En cuanto salga... = As soon as I leave...

You do not use the future tense after en cuanto in this kind of sentence.

A very common pattern is:

• En cuanto + present subjunctive, future / command / another future idea

For example:

• En cuanto llegue, te llamaré.
• As soon as I arrive, I’ll call you.

If the action is habitual or already real, then Spanish can use the indicative instead:

• En cuanto salgo de clase, voy a casa.
• As soon as I leave class, I go home. / Whenever I leave class, I go home.

Here en cuanto means as soon as.

It introduces the moment when the next action happens:

• En cuanto salga de la facultad, pasaré...
• As soon as I leave the university/faculty, I’ll stop by...

It is a very common time expression. Other similar expressions are:

• tan pronto como
• apenas

But en cuanto is very natural and common in everyday Spanish.

It is in the subjunctive because the speaker is referring to an action that is:

• in the future
• not completed yet
• viewed as pending

After time expressions like cuando, en cuanto, hasta que, tan pronto como, Spanish often uses the present subjunctive when the action is still in the future.

That is why:

• En cuanto salga de la facultad... is correct.

Compare:

• Cuando salgo de la facultad, siempre estoy cansado.
  Habitual action → indicative
• Cuando salga de la facultad, te llamaré.
  Future action → subjunctive

Not exactly. This is a classic false friend.

In this sentence, la facultad usually means:

• a university faculty or school, such as the Faculty of Law
• and often, by extension, the university building/department where you study

It does not usually mean the teaching staff in the English sense of faculty.

So in everyday Spain Spanish, salir de la facultad often means something like:

• leaving your university building
• leaving your faculty/school at university

Because the verb salir often takes de to show the place you are leaving:

• salir de casa = to leave home
• salir del trabajo = to leave work
• salir de la facultad = to leave the university/faculty

So salir de is the normal pattern when you mean to go out of / to leave a place.

Pasar por here means to stop by, to go by, or to drop in at a place, often briefly and often on the way somewhere.

So:

• pasaré por la papelería = I’ll stop by the stationery shop / copy shop

It does not necessarily mean entering for a long time. It suggests a quick visit, often because it is convenient.

This is slightly different from just:

• iré a la papelería = I’ll go to the stationery shop

Pasar por often sounds more like I’ll swing by.

In Spain, papelería is usually a stationery shop, but in practice it often also sells things like:

• pens
• notebooks
• paper
• school supplies
• printer-related items

And very often, depending on the shop, it may also offer services like:

• photocopies
• printing
• document binding

So in this sentence, going to a papelería to make a photocopy sounds very natural in Spain.

Yes. Hacer una fotocopia is completely natural and very common.

It means:

• to make a photocopy

Spanish often uses hacer in places where English might choose a more specific verb.

You may also hear:

• fotocopiar un documento = to photocopy a document

But hacer una fotocopia is an extremely normal everyday phrase.

Because para + infinitive is the standard way to express purpose in Spanish.

So:

• para hacer una fotocopia = in order to make a photocopy / to make a photocopy

This structure is very common:

• Voy al banco para sacar dinero.
• He called to ask a question. → Llamó para hacer una pregunta.

So here, para hacer una fotocopia explains why the speaker will stop by the shop.

Spanish uses articles differently from English.

• la facultad and la papelería use the definite article because they refer to identifiable places in the situation: the faculty/university building and the stationery shop.
• una fotocopia uses the indefinite article because it means a photocopy, not a specific previously mentioned one.

Spanish often uses definite articles where English might sometimes leave them out.

Yes, in some contexts Spanish can use the present tense to talk about a planned future action:

• En cuanto salga de la facultad, paso por la papelería...

That can sound natural in conversation, especially for near-future plans.

However, pasaré is clearer and more explicit as a future action. It sounds very standard and neutral.

So both are possible, but:

• pasaré = explicitly future
• paso = present used with future meaning, often more conversational

Yes, that is also correct.

You can put the time clause first:

• En cuanto salga de la facultad, pasaré por la papelería...

Or second:

• Pasaré por la papelería en cuanto salga de la facultad.

Both are natural. Putting en cuanto salga de la facultad first gives slightly more emphasis to the time condition: as soon as I get out...

Yes, a couple of things feel very Spain-oriented:

• la facultad is a very common way in Spain to refer to one’s university faculty/building.
• la papelería as the place to get a photocopy is also very natural in Spain.

Also, fotocopia is widely understood everywhere, but the whole situation sounds very typical of Spain Spanish daily life: leaving university and stopping by a papelería for a copy.