El comentario parece estúpido.

Because comentario is a masculine noun in Spanish, so it takes the masculine singular article el.

A useful pattern is that many nouns ending in -o are masculine:

• el libro
• el coche
• el comentario

It is not a perfect rule, but it works very often.

Because the subject is el comentario, which is singular.

The verb parecer must agree with the subject:

• El comentario parece estúpido. = one comment
• Los comentarios parecen estúpidos. = more than one comment

So parece is the third-person singular form.

Here, parece means seems or appears to be.

So the sentence is saying that the comment gives the impression of being stupid, not necessarily stating it as an absolute fact.

That is different from:

• El comentario es estúpido. = The comment is stupid.
• El comentario parece estúpido. = The comment seems stupid.

Using parece sounds a bit less direct and a bit less absolute.

In Spanish, parecer can be followed directly by an adjective.

So:

• parece estúpido literally works as
• seems stupid

You can also say:

• parece ser estúpido

But that version is usually more formal, heavier, or less natural in everyday speech. In normal conversation, parece estúpido is the usual choice.

Because estúpido agrees with comentario, which is masculine singular.

In Spanish, adjectives usually match the noun they describe in gender and number:

• el comentario estúpido / parece estúpido
• la idea estúpida / parece estúpida
• los comentarios estúpidos
• las ideas estúpidas

Even though the adjective comes after parece, it still agrees with the subject.

Grammatically, it describes el comentario.

So the sentence says the comment seems stupid, not necessarily the person seems stupid.

Of course, in real life, calling someone's comment stupid may imply criticism of the person too, but the grammar points only to comentario.

Yes. That is grammatical, but it means something slightly different.

• El comentario parece estúpido. = The comment seems stupid.
• El comentario es estúpido. = The comment is stupid.

Using es is stronger and more direct. Using parece sounds more cautious, more subjective, or less confrontational.

The accent mark shows where the stress goes: es-TÚ-pi-do.

Without the accent, Spanish stress rules would make you pronounce it differently. The written accent tells you that the stressed syllable is tú.

So:

• estúpido = stress on TÚ

This is very important for correct pronunciation.

A common Spain pronunciation would be:

el ko-men-TA-rio pa-RE-ce es-TÚ-pi-do

More approximately in IPA: [el komentáɾjo paɾéθe estúpido] in much of Spain

A few notes:

• comentario has a normal k sound at the start.
• parece often has a th sound for ce in most of Spain, like pa-RE-the.
• estúpido is stressed on TÚ.

In many parts of Latin America, ce would sound like s instead.

It can be.

Estúpido usually means stupid, so it is clearly negative and can sound harsh, especially if you use it about a person.

With comentario, it criticizes the comment rather than directly insulting a person, but it can still sound blunt.

Softer alternatives might be:

• parece tonto = seems silly / dumb
• parece poco inteligente = seems not very intelligent
• parece absurdo = seems absurd

So yes, estúpido is natural Spanish, but it is not especially polite.

Normally, no.

In standard Spanish, you would usually say:

• El comentario parece estúpido.

Dropping the article sounds incomplete in an ordinary sentence. You might see article-dropping in headlines, notes, or very special contexts, but not in normal everyday grammar.

So for a learner, the safe and correct version is:

• El comentario parece estúpido.