Ella parece interesada.

Because parece is the he/she/it form of the verb parecer in the present tense.

• parecer = to seem / to appear
• ella parece = she seems

A quick pattern:

• yo parezco = I seem
• tú pareces = you seem
• él/ella parece = he/she seems
• nosotros parecemos = we seem
• vosotros parecéis = you all seem
• ellos parecen = they seem

So Ella parece interesada literally follows the pattern She seems interested.

Because it describes ella, and Spanish adjectives usually agree with the person or thing they describe in gender and number.

Since ella is feminine singular, the adjective must also be feminine singular:

• interesado = masculine singular
• interesada = feminine singular
• interesados = masculine/mixed plural
• interesadas = feminine plural

Examples:

• Él parece interesado. = He seems interested.
• Ella parece interesada. = She seems interested.
• Ellos parecen interesados. = They seem interested.
• Ellas parecen interesadas. = They seem interested.

Yes. Very often you can simply say:

• Parece interesada.

Spanish often drops subject pronouns because the verb ending already tells you who the subject is. However, in this sentence, dropping ella can sometimes make the subject less clear unless context makes it obvious.

You might keep ella when:

• you want to emphasize she
• you are contrasting her with someone else
• you want to make the subject especially clear

For example:

• Él está aburrido, pero ella parece interesada.

This is a very common question.

• Ella parece interesada = She seems interested
• Ella está interesada = She is interested

So:

• parece gives an impression or appearance
• está states it more directly as a fact or condition

Examples:

• Parece interesada, pero no estoy seguro.
  She seems interested, but I’m not sure.

• Está interesada en el curso.
  She is interested in the course.

So if you are observing someone and making a guess, parece is a natural choice.

Because interesado/interesada can mean different things depending on context, and ser changes the meaning.

• estar interesado/a = to be interested
• ser interesado/a = to be self-interested, calculating, or motivated by personal gain

So:

• Ella está interesada = She is interested.
• Ella es interesada = She is selfishly interested / she has ulterior motives.

That is a big difference. For the meaning of interested, Spanish normally uses estar, not ser.

It is functioning as an adjective here.

Historically, forms like interesado/interesada come from participles, but in everyday grammar here it works like an adjective describing a person’s state or appearance.

In Ella parece interesada:

• parece = the verb
• interesada = adjective describing ella

You can think of it the same way as:

• Ella parece cansada. = She seems tired.
• Ella parece contenta. = She seems happy.

Yes, it is very similar:

• Ella parece interesada.
• Parece que está interesada.

Both mean roughly She seems interested.

The difference is mostly structural:

• parece interesada = seem + adjective
• parece que está interesada = it seems that she is interested

The second version can sound a little more explicit or slightly fuller. Both are natural.

Not in this sentence by itself, but you often do if you say what she is interested in.

• Ella parece interesada. = She seems interested.
• Ella parece interesada en la historia. = She seems interested in history.
• Ella está interesada en el proyecto. = She is interested in the project.

So en is used with the thing that interests her.

Here interesada comes after the verb because it is a subject complement: it describes the subject after a verb like parecer.

This is normal in Spanish:

• Ella parece cansada.
• Él está feliz.
• Nosotros somos altos.

It is not about adjective position before or after a noun. This is different from something like:

• una mujer interesante = an interesting woman

In Ella parece interesada, the adjective is not directly attached to a noun; it completes the idea after the verb.

In standard Peninsular Spanish, a rough guide is:

• Ella = EH-ya
• parece = pa-RE-the
• interesada = in-te-re-SA-da

A few key points for Spain:

• ll is usually pronounced like y
• ce in parece is often pronounced with the th sound of thin in much of Spain
• the stress falls on:
  • pa-RE-ce
  • in-te-re-SA-da

So a rough English-friendly version is:

EH-ya pa-RE-the in-te-re-SA-da

No. They are different verbs.

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

Compare:

• Ella parece interesada. = She seems interested.
• Ella se parece a su madre. = She looks like her mother.

So in your sentence, the correct verb is parecer, not parecerse.

Yes, in many contexts you can, but it is not exactly identical.

• Ella parece interesada = She seems interested.
• Ella se ve interesada = She looks interested.

Difference:

• parece can refer more generally to the impression she gives
• se ve focuses more on how she looks

In many everyday situations, both work. But parece is often the more neutral and flexible choice.

It is neutral and works in both formal and informal situations.

• talking to friends: Ella parece interesada.
• in a meeting: Ella parece interesada en la propuesta.
• in writing: also fine

Nothing in the sentence is especially slangy, literary, or overly formal. It is a standard, natural sentence.