La profesora les explica la idea a los estudiantes en clase.

Because Spanish very often uses an indirect object pronoun even when the full indirect object is also stated.

In this sentence:

• les = to them
• a los estudiantes = to the students

So both point to the same people. This is called indirect object doubling, and it is very common and natural in Spanish.

So:

• La profesora les explica la idea a los estudiantes. = The teacher explains the idea to the students.

Even though English would not usually repeat to the students and them, Spanish often does.

Here, les is an indirect object pronoun meaning to them.

The structure is:

• explicar algo a alguien = to explain something to someone

So in this sentence:

• la idea = the thing being explained
• les / a los estudiantes = the people receiving the explanation

That means:

• la idea is the direct object
• les / a los estudiantes is the indirect object

The verb explicar is used with this pattern:

• explicar algo a alguien
• to explain something to someone

So the a marks the person who receives the explanation.

Examples:

• Explico la lección a María. = I explain the lesson to María.
• Explican el problema a los alumnos. = They explain the problem to the students.

This is not the same as the personal a used with direct objects. Here, a los estudiantes is an indirect object, so the a belongs to the verb pattern.

Because the thing being explained is the idea.

Think of the verb pattern:

• Someone explains something to someone

In this sentence:

• La profesora = the subject
• explica = the verb
• la idea = what is explained → direct object
• a los estudiantes / les = who receives it → indirect object

A good test in English is:

• What is the teacher explaining? The idea.
• To whom is she explaining it? To the students.

Spanish often uses the definite article with professions or roles when talking about a specific person.

So:

• La profesora = The teacher

If you just said profesora on its own, that would usually not work here as a normal subject.

Compare:

• La profesora habla. = The teacher speaks.
• Es profesora. = She is a teacher.

Notice the difference:

• with ser to say someone’s profession, you usually do not use the article
• as the subject the teacher, you usually do use the article

Explica is the present tense: she explains or she is explaining, depending on context.

Spanish often uses the simple present where English might use either:

• She explains the idea to the students in class
• She is explaining the idea to the students in class

If you specifically wanted to emphasize the ongoing action right now, Spanish could also say:

• La profesora está explicando la idea a los estudiantes en clase.

But explica is perfectly natural.

In many cases, especially in standard modern Spanish, it is most natural to keep les when the indirect object is mentioned.

So:

• La profesora les explica la idea a los estudiantes. = very natural

Without les:

• La profesora explica la idea a los estudiantes.

This is understandable and grammatical, but the version with les is usually more typical in everyday Spanish.

Yes, Spanish word order is more flexible than English.

The basic version here is:

• La profesora les explica la idea a los estudiantes en clase.

But these are also possible in the right context:

• La profesora explica la idea a los estudiantes en clase.
• A los estudiantes les explica la idea la profesora.

Changing the order can change focus or emphasis, but not the core meaning.

For learners, the given order is a very safe and natural one: subject + pronoun + verb + direct object + indirect object + place

En clase is a very common fixed expression meaning in class or during class.

So:

• en clase = in class / during class

If you say en la clase, it usually sounds more like in the classroom or in the specific class depending on context.

Compare:

• Los estudiantes hablan en clase. = The students speak in class.
• Los estudiantes están en la clase. = The students are in the classroom / in the class.

So in your sentence, en clase is the natural general expression.

Because idea is a feminine noun in Spanish, even though it ends in -a, which fits the usual pattern.

So you get:

• la idea
• una idea
• la buena idea

The adjective must agree with it:

• una idea interesante
• la idea correcta

This is just something you learn with the noun: idea is feminine.

Normally, les is plural: to them.

However, in very formal usage, les can also refer to you in the plural (ustedes) or sometimes appear in polite contexts. But in this sentence, because we have a los estudiantes, it clearly means to the students / to them.

For singular to him / to her / to you (formal singular), Spanish usually uses:

• le

Examples:

• La profesora le explica la idea al estudiante. = The teacher explains the idea to the student.
• La profesora les explica la idea a los estudiantes. = The teacher explains the idea to the students.

Then you get both a direct object pronoun and an indirect object pronoun.

Start with:

• La profesora les explica la idea a los estudiantes.

Replace:

• la idea → la
• a los estudiantes / les → les

You might expect les la, but Spanish changes le/les to se before lo/la/los/las.

So the result is:

• La profesora se la explica.

This means:

• The teacher explains it to them.

This is a very important Spanish pronoun rule.

Sometimes, but the meaning changes a bit.

• explicar = to explain
• enseñar = to teach / to show

In this sentence, explica la idea focuses on explaining the idea clearly.

If you say:

• La profesora enseña la idea...

that sounds less natural in many contexts, because in Spanish enseñar is more often used for teaching a subject/skill or showing something.

So here explica is the best choice.

Yes. La profesora is feminine, so it refers to a female teacher.

If it were a male teacher, you would say:

• El profesor les explica la idea a los estudiantes en clase.

Only the noun and article change:

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

The rest of the sentence stays the same.