Prefiero un correo breve y formal cuando escribo a una profesora que no conozco.

In Spanish, subject pronouns are often omitted because the verb ending already shows who the subject is.

• prefiero = I prefer
• prefieres = you prefer
• prefiere = he/she/you(formal) prefers

So Prefiero un correo... is completely natural. You could say Yo prefiero... if you want extra emphasis, contrast, or clarity.

Because preferir is a stem-changing verb. In the present tense, the e in the stem changes to ie in most forms:

• yo prefiero
• tú prefieres
• él/ella prefiere
• nosotros preferimos
• vosotros preferís
• ellos prefieren

This is a very common pattern in Spanish.

In Spain, un correo very often means an email, as a short form of correo electrónico.

So in this sentence, un correo breve y formal most naturally means a brief, formal email.

Depending on context, correo can also refer more generally to mail/post, but here escribo a una profesora strongly suggests email.

Both can mean short, but breve is often more natural in formal or professional contexts.

• un correo breve = a brief email
• un correo corto = a short email

Corto is not wrong, but breve sounds more polished and appropriate for formal writing. That is why it fits well with formal in this sentence.

In Spanish, descriptive adjectives usually come after the noun.

• un correo breve
• una profesora amable
• una carta formal

Putting adjectives before the noun is sometimes possible, but it often adds emphasis, sounds more literary, or slightly changes the nuance. In this sentence, the normal order is correo breve y formal.

Because correo is a masculine noun in Spanish, so it takes the masculine article un.

• el correo
• un correo

The ending -o often suggests a masculine noun, and in this case that pattern works.

Because with escribir, the person you write to is introduced by a.

• Escribo un correo = I write an email
• Escribo a una profesora = I write to a teacher

So in the full sentence:

• un correo is the thing being written
• a una profesora is the person receiving it

This a is very important and is the natural structure in Spanish.

Una profesora means a teacher, not a specific teacher already identified in the conversation.

Use una when the person is not specifically defined or is being mentioned for the first time. Use la when both speaker and listener already know which teacher is meant.

So:

• una profesora = a teacher / some teacher
• la profesora = the teacher / the specific teacher

Because profesora is feminine and refers to a female teacher. Spanish nouns for people often change according to gender:

• profesor = male teacher
• profesora = female teacher

Since the sentence refers to a female teacher, profesora is the correct form.

Because in Spanish, conocer is used for knowing people, being familiar with them, or having met them.

• No conozco a esa profesora = I don’t know that teacher
• No sé su nombre = I don’t know her name

So:

• conocer = know a person / be acquainted with
• saber = know a fact / know information / know how to do something

That is why que no conozco is the natural choice here.

In standard Spanish, no normally goes directly before the conjugated verb.

• no conozco
• no escribo
• no prefiero

So que no conozco literally follows the usual Spanish negation pattern: that I do not know.

Because the sentence describes a habitual or general action: what the speaker usually prefers when writing to a teacher they don’t know.

So cuando escribo means when I write / whenever I write.

Spanish often uses the present tense this way for repeated or general situations.

Yes. Cuando escriba would usually refer to a future action.

Compare:

• cuando escribo a una profesora que no conozco = when I write / whenever I write to a teacher I don’t know
• cuando escriba a una profesora que no conozco = when I write to a teacher I don’t know in the future

So:

• present indicative after cuando = habitual, repeated, general
• present subjunctive after cuando = future or not-yet-realised action

It describes una profesora.

So the structure is:

• una profesora
• que no conozco = whom I don’t know

This is a relative clause, introduced by que. It gives extra information about the teacher.

You can think of it as:

• a teacher
• that / whom I don’t know

Yes, many Spanish speakers understand email, e-mail, or mail, but in Spain correo or correo electrónico is often the more standard and natural choice, especially in careful or neutral Spanish.

So for a learner aiming at Spanish from Spain, correo is an excellent choice here.