El edificio parece feo.

In Spanish, a singular countable noun usually needs some kind of determiner, such as:

• el = the
• un = a
• este = this

So El edificio is the normal way to say the building.

Just saying Edificio parece feo would sound incomplete or unnatural in standard Spanish.

Because edificio is a masculine noun in Spanish. Nouns have grammatical gender, and edificio takes:

• el edificio
• un edificio

That masculine gender also affects the adjective:

• el edificio feo
• not el edificio fea

A useful pattern is that many nouns ending in -o are masculine, though there are exceptions in Spanish overall.

Parece is the third-person singular present tense of parecer.

The subject is el edificio, which is singular, so the verb must also be singular:

• el edificio parece
• los edificios parecen

So parece is used because the building is a singular subject.

Spanish often leaves out subject pronouns when they are not needed.

In English, you must say It looks ugly.
In Spanish, the subject is already clear from el edificio, so there is no need for a separate it.

Spanish usually avoids unnecessary subject pronouns unless you want contrast or emphasis.

They are close, but not identical.

• parece feo = it seems / looks ugly
• es feo = it is ugly

Parecer is a little softer and more about appearance or impression.
Ser is more direct and sounds more like a statement of fact.

So:

• El edificio parece feo = it looks ugly
• El edificio es feo = it is ugly

Yes, but the meaning shifts slightly.

• es feo = ugly by nature or generally ugly
• está feo = ugly-looking right now, or in a certain condition
• parece feo = seems/looks ugly

For example, está feo could be used if the building looks bad today because it is dirty, damaged, or badly lit.

So parece, es, and está are not interchangeable in every situation.

Because adjectives in Spanish agree with the noun in gender and number.

Here:

• edificio = masculine singular
• so the adjective must be feo = masculine singular

Compare:

• el edificio feo
• la casa fea
• los edificios feos
• las casas feas

Yes. This is a very common pattern in Spanish:

• parecer + adjective

Examples:

• parece interesante
• parece caro
• parecen difíciles

So parece feo is a completely normal structure. No extra word is needed between the verb and the adjective.

No. They are different verbs.

• parecer = to seem / to look
• parecerse a = to resemble / to look like

So:

• El edificio parece feo = the building seems ugly
• El edificio se parece a una fábrica = the building looks like a factory

In your sentence, the correct verb is parecer, not parecerse.

In Spain, parece is pronounced roughly pa-RE-the, because ce is usually pronounced like the th in think.

A rough pronunciation guide for the whole sentence in Spain Spanish:

• El edificio ≈ el eh-dee-FI-thyo
• parece ≈ pa-RE-the
• feo ≈ FEH-o

A couple of useful details:

• edificio is stressed on fi
• feo has two syllables: fe-o, not one

So the full sentence sounds roughly like:

el eh-dee-FI-thyo pa-RE-the FEH-o