El profesor usa un subrayador amarillo para resaltar la idea principal.

In Spanish, descriptive adjectives usually come after the noun.

So:

• un subrayador amarillo = a yellow highlighter

This is the normal word order. Putting the color before the noun would sound unusual in everyday speech.

A few more examples:

• una camisa roja = a red shirt
• un libro interesante = an interesting book
• una mesa grande = a big table

Subrayador means a tool used for underlining or highlighting text. In many places, it can mean highlighter.

However, vocabulary varies a lot across Latin America. Depending on the country, people may also say:

• resaltador
• marcador fluorescente
• sometimes just marcador

So subrayador is understandable, but it is not the only possible word.

Usa is the third-person singular present tense of usar.

• yo uso = I use
• tú usas = you use
• él / ella / usted usa = he / she / you use

Since the subject is El profesor (The teacher), the verb must be usa.

So:

• El profesor usa... = The teacher uses...

After para to express purpose or in order to, Spanish uses the infinitive.

So:

• para resaltar = to highlight / in order to highlight

This is the same pattern you see in:

• Estudio para aprender. = I study to learn.
• Uso gafas para leer. = I use glasses to read.
• Necesito dinero para viajar. = I need money to travel.

So para + infinitive is a very common structure.

They are related, but not exactly the same.

• subrayar literally means to underline
• resaltar means to highlight, emphasize, or make stand out

In this sentence, the teacher uses a subrayador but does so para resaltar the main idea.

That makes sense because a highlighter is a tool used to make important information stand out.

So the sentence is basically using:

• the tool: subrayador
• the purpose: resaltar la idea principal

Spanish often uses articles more regularly than English does.

Here, la idea principal means:

• the main idea

Because it refers to a specific idea in the text, Spanish naturally uses the definite article la.

Compare:

• Resalta la idea principal. = Highlight the main idea.
• Busca la respuesta correcta. = Look for the correct answer.

Leaving out the article would usually sound incomplete or unnatural here.

Both are possible, but they mean slightly different things.

• el profesor = the teacher
• un profesor = a teacher

El profesor suggests a specific teacher, or the teacher in a known context, such as a classroom example.

Textbooks often use el in model sentences because they imagine a definite person in the scene.

So this sentence sounds like:

• The teacher uses a yellow highlighter...

not just any teacher in general.

Normally:

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

So if the teacher is female, the sentence would usually become:

• La profesora usa un subrayador amarillo para resaltar la idea principal.

Spanish nouns for professions often change form depending on gender.

This is about indefinite vs. definite articles.

• un subrayador = a highlighter
  This introduces the object as one example of that kind of item.
• la idea principal = the main idea
  This refers to a specific idea, the main one.

So the sentence combines:

• a yellow highlighter
• the main idea

That matches English pretty closely in this case.

Yes. The Spanish present tense often covers both meanings:

• a habitual action: The teacher uses a yellow highlighter
• an action happening right now, depending on context

So usa can mean:

• uses
• is using

If you wanted to make the action clearly progressive, you could say:

• El profesor está usando un subrayador amarillo...

But the simple present usa is very normal and common.

A very common alternative would be:

• El profesor usa un resaltador amarillo para resaltar la idea principal.

In some places, that may sound more natural than subrayador.

You might also hear:

• El profesor usa un marcador amarillo para resaltar la idea principal.
• El profesor usa un marcador fluorescente amarillo...

The grammar stays the same; the main difference is regional vocabulary.