Después de haber abierto la ventana, me siento más tranquilo.

Both are possible.

• Después de abrir la ventana... = After opening the window...
• Después de haber abierto la ventana... = After having opened the window...

The version with haber + past participle emphasizes that the action of opening was already completed before the next action or state. In everyday Spanish, después de abrir is often more common because it is simpler, but después de haber abierto is perfectly correct and slightly more explicit.

Haber abierto is the perfect infinitive.

It is formed with:

• haber
  • past participle

So here:

• haber = infinitive of to have (auxiliary verb)
• abierto = past participle of abrir

This structure expresses a completed action in a non-finite form: to have opened / having opened.

Because abrir has an irregular past participle.

• regular pattern would suggest something like abrido
• but the correct form is abierto

Some common irregular past participles are:

• abrir → abierto
• decir → dicho
• escribir → escrito
• hacer → hecho
• poner → puesto
• ver → visto
• volver → vuelto

So haber abierto is the correct form.

Yes, absolutely.

• Después de que abrí la ventana, me siento más tranquilo.
• Después de haber abierto la ventana, me siento más tranquilo.

Both are correct, but they are structured differently:

• después de que + verb uses a full clause
• después de + infinitive / perfect infinitive uses a non-finite construction

The version with después de que abrí sounds a bit more direct and explicit about the subject. The infinitive version is very natural when the subject is understood.

Because sentirse means to feel, and that is exactly what the sentence is expressing.

• me siento más tranquilo = I feel calmer

Compare:

• soy tranquilo = I am a calm person by nature
• estoy tranquilo = I am calm right now
• me siento tranquilo = I feel calm

So me siento focuses on the speaker’s internal feeling.

Más tranquilo means calmer or more at ease.

It does not mean simply better in a general sense. It suggests that opening the window made the speaker feel less tense, less bothered, or more relaxed.

So:

• me siento más tranquilo = I feel calmer / more relaxed

Because adjectives in Spanish agree with the person they describe.

If the speaker is male, you would say:

• me siento más tranquilo

If the speaker is female, you would say:

• me siento más tranquila

If more than one person is speaking:

• nos sentimos más tranquilos = masculine or mixed group
• nos sentimos más tranquilas = all female group

Because Spanish usually omits subject pronouns when the verb already makes the subject clear.

• me siento already tells us the subject is I
• so yo is not necessary

You could say:

• Yo me siento más tranquilo

but that would usually add emphasis, contrast, or clarification. In neutral speech, Spanish normally leaves yo out.

Because sentirse is a reflexive verb.

The full verb is:

• sentirse = to feel

So the reflexive pronoun changes depending on the subject:

• me siento = I feel
• te sientes = you feel
• se siente = he/she feels
• nos sentimos = we feel

In this sentence, me matches the first-person subject.

Because when a past participle is used with haber, it does not agree in gender or number.

So we say:

• he abierto la ventana
• he abierto las ventanas
• después de haber abierto la ventana

The form stays abierto.

Agreement happens in other structures, for example with adjectives:

• la ventana está abierta
• las ventanas están abiertas

But with haber, the participle stays invariable.

Yes. Spanish allows flexible word order.

You can say:

• Después de haber abierto la ventana, me siento más tranquilo.
• Me siento más tranquilo después de haber abierto la ventana.

Both are natural. The first version puts the time/action background first. The second starts with the main idea: I feel calmer.

Después de means after when it is followed by a noun or an infinitive-type structure.

Examples:

• después de la cena = after dinner
• después de abrir la ventana = after opening the window
• después de haber abierto la ventana = after having opened the window

If you use a full verb clause, then Spanish usually uses después de que:

• después de que abrí la ventana

So:

• después de + noun / infinitive
• después de que + conjugated verb

They are simply the correct standard spellings:

• después
• más

The accent marks are part of the written forms of these words and help indicate correct stress and distinguish meanings.

For más, the accent is especially important because:

• más = more
• mas = but (very literary/old-fashioned)

So in this sentence, más tranquilo must have the accent because it means more calm / calmer.

Yes, it sounds natural and correct in Spain.

A speaker in Spain might also choose simpler alternatives, depending on context:

• Después de abrir la ventana, me siento más tranquilo.
• Ahora que he abierto la ventana, me siento más tranquilo.
• Me siento más tranquilo después de abrir la ventana.

But your original sentence is fully natural and grammatically solid in Spain Spanish.