La profesora quiere aclarar una duda importante antes del examen oral.

Because profesora is the feminine form of profesor. Spanish nouns that refer to people often change form depending on gender:

• el profesor = the male teacher
• la profesora = the female teacher

The article changes too:

• el for masculine singular nouns
• la for feminine singular nouns

So la profesora means the teacher is female.

After a conjugated verb like quiere (she wants), Spanish usually uses an infinitive for the next verb:

• quiere aclarar = wants to clarify

Here:

• quiere is the conjugated form of querer
• aclarar stays in the infinitive

This is the same pattern as:

• Quiero comer = I want to eat
• Necesita estudiar = He/She needs to study
• Prefieren salir = They prefer to go out

So quiere aclarar is the normal structure.

Quiere is the third person singular form of querer in the present tense. It can mean:

• he wants
• she wants
• it wants
• you want (formal usted)

In this sentence, because the subject is la profesora, it means she wants.

A few present-tense forms of querer are:

• yo quiero
• tú quieres
• él/ella/usted quiere
• nosotros queremos
• vosotros queréis
• ellos/ellas/ustedes quieren

Here aclarar means to clarify, to clear up, or to explain so that something becomes clear.

It can be used in different contexts:

• aclarar una duda = clarify a doubt / clear up a question
• aclarar un malentendido = clear up a misunderstanding
• aclarar algo = make something clear

So in this sentence, the teacher wants to make something clear before the oral exam.

Yes, duda literally means doubt, but in Spanish it is also very commonly used to mean a question, uncertainty, or thing someone is unsure about.

So aclarar una duda is a very natural expression meaning:

• clarify a doubt
• clear up a question
• resolve an uncertainty

A student might say:

• Tengo una duda = I have a question / I’m unsure about something

So even though doubt is the literal word, the meaning in context is often closer to question or uncertainty.

In Spanish, adjectives usually come after the noun:

• una duda importante = an important doubt/question
• un examen oral = an oral exam

Putting the adjective after the noun is the most neutral and common order.

Sometimes adjectives can go before the noun, but that often changes the tone, emphasis, or style. For learners, the safest default is:

• noun + adjective

So una duda importante is the normal word order.

Because de + el contracts to del in Spanish.

So:

• de el examen ❌
• del examen ✅

This contraction is mandatory in normal Spanish, except when el is part of a proper name, such as:

• de El Escorial ✅

In your sentence:

• antes del examen oral = before the oral exam

The expression is antes de + noun / infinitive.

Examples:

• antes de salir = before leaving
• antes de la clase = before class
• antes del examen = before the exam

In your sentence, antes de is followed by el examen oral, and because de + el = del, you get:

• antes del examen oral

So the structure is:

• antes de + noun
• antes de + infinitive

Again, this is the usual Spanish pattern: noun + adjective.

• examen oral = oral exam
• clase práctica = practical class
• prueba escrita = written test

In English, many adjectives come before the noun, but in Spanish they often come after it.

So:

• English: oral exam
• Spanish: examen oral

Examen is masculine:

• el examen
• un examen

Its plural is:

• los exámenes

You cannot always rely only on the ending to know gender. Although many nouns ending in -a are feminine and many ending in -o are masculine, nouns ending in -en, -ma, -ista, and others often need to be learned individually.

So it is best to memorize nouns together with their article:

• la duda
• el examen
• la profesora

Because the direct object here is una duda importante, which is a thing, not a person.

Spanish uses the personal a before a specific person (or sometimes a pet/personified being) when that person is the direct object:

• Veo a la profesora = I see the teacher
• Ayudo a mi amigo = I help my friend

But with things, there is normally no personal a:

• aclarar una duda
• comprar un libro
• hacer un examen

So aclarar una duda importante is correct without a.

Yes. Depending on context, profesor/profesora can mean:

• teacher
• professor

In everyday Spanish, it is often used for school teachers as well as university instructors. The exact English translation depends on context.

So la profesora could be:

• the teacher
• the professor

If the meaning has already been shown to the learner, context will decide which English word is best.

Yes, and it would sound natural, but the nuance is slightly different.

• aclarar una duda = clarify a doubt / clear up a question
• resolver una duda = resolve a doubt / answer a question

Aclarar focuses on making something clear.
Resolver focuses a bit more on solving it completely.

Both are common, but aclarar una duda is a very natural and idiomatic choice here.