La teoría parece correcta.

Because teoría is a singular feminine noun, so it normally takes the feminine singular definite article la when you mean the theory.

• la teoría = the theory
• el libro = the book
• las teorías = the theories

In this sentence, la teoría is the subject: the thing that seems correct.

The accent mark in teoría shows where the stress falls: te-o-RÍ-a.

This word has a hiatus: í + a are pronounced in separate syllables instead of blending together. The written accent helps show that pronunciation.

So:

• teoría = te-o-rí-a (4 syllables)

This is important both for correct pronunciation and correct spelling.

Parece means seems or appears.

It comes from the verb parecer, which is often used to give an impression rather than a certainty.

• La teoría parece correcta. = The theory seems correct.
• Parece interesante. = It seems interesting.

So the speaker is not saying the theory is definitely correct, only that it appears to be.

Because parece is the third-person singular form of parecer in the present tense, and the subject la teoría is singular.

Present tense of parecer:

• yo parezco
• tú pareces
• él / ella / usted parece
• nosotros/as parecemos
• vosotros/as parecéis
• ellos / ellas / ustedes parecen

Since la teoría = it / she grammatically, you use parece.

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

• teoría is feminine singular
• so the adjective must also be feminine singular: correcta

Compare:

• el libro parece correcto = the book seems correct
• la teoría parece correcta = the theory seems correct
• las teorías parecen correctas = the theories seem correct

It describes teoría, not parece.

In this kind of sentence, parecer works like a linking verb, similar to to seem in English. The adjective tells you something about the subject.

So the structure is:

• La teoría = subject
• parece = linking verb
• correcta = adjective describing the subject

In other words, the theory is what seems correct.

Yes, but the meaning is a bit different.

• La teoría parece correcta = The theory seems correct
  This sounds less certain, more like an impression.
• La teoría es correcta = The theory is correct
  This sounds more definite.

So parece adds uncertainty or caution.

Usually, no, not in the same sense.

• es correcta = it is correct
• parece correcta = it seems correct

Using estar correcta is much less natural here in standard general Spanish. Correcto/a is usually used with ser when talking about whether something is right or correct.

So for this sentence:

• natural: La teoría parece correcta
• also natural: La teoría es correcta
• less appropriate here: La teoría está correcta

Because Spanish usually does not need a subject pronoun when the subject is already stated.

Here, la teoría is the subject, so there is no need to add a separate word for it.

English:

• The theory seems correct.

Spanish:

• La teoría parece correcta.

Spanish often leaves out pronouns like it, he, or she when the meaning is already clear.

Yes. Parecer can be used in more than one pattern.

Two common ones are:

1. parecer + adjective

   • La teoría parece correcta.
   • The theory seems correct.

2. parecer + infinitive

   • La teoría parece funcionar.
   • The theory seems to work.

So in your sentence, parece correcta is the pattern seems + adjective.

A careful Spain Spanish pronunciation would be approximately:

la te-o-RÍ-a pa-RE-ce ko-REK-ta

A bit more specifically:

• la = lah
• teoría = te-o-RÍ-a
• parece = pa-RE-ce
• correcta = ko-RREK-ta

In much of Spain:

• c before e in parece is pronounced like the th in thin
• so parece sounds roughly like pa-RE-the

That gives:

• la te-o-RÍ-a pa-RE-the ko-RREK-ta

Usually, La teoría parece correcta is the most neutral and natural order.

Spanish can sometimes change word order for emphasis, but in a simple sentence like this, changing it would usually sound marked or less natural.

So for ordinary use, keep:

• La teoría parece correcta.

That is the standard subject + verb + adjective structure here.

The plural version is:

Las teorías parecen correctas.

Here everything changes to match the plural subject:

• la → las
• teoría → teorías
• parece → parecen
• correcta → correctas

So Spanish agreement affects the article, the verb, and the adjective.