Después de haber corrido en la playa, me siento más tranquilo.

Because after the preposition de, Spanish uses an infinitive, not a conjugated verb.

• haber corrido = to have run
• he corrido = I have run

So:

• después de haber corrido = after having run
• después de he corrido is ungrammatical

This structure is called the perfect infinitive: haber + past participle. It shows that the action of running happened before the action in the main clause.

Yes. That is very natural, and in many situations it is the more common way to say it.

Compare:

• Después de correr en la playa, me siento más tranquilo.
• Después de haber corrido en la playa, me siento más tranquilo.

Both are correct.

The difference is mainly one of emphasis:

• después de correr = after running
• después de haber corrido = after having run

The version with haber corrido makes the idea of completed action a little more explicit.

It is used to show that the running happened before the feeling.

The timeline is:

1. I ran on the beach.
2. Now I feel calmer.

Spanish can express that earlier completed action with haber + past participle:

• haber corrido = to have run

So the sentence highlights that the calm feeling comes after the completed run.

Because sentirse means to feel in the sense of to feel a certain way.

• me siento tranquilo = I feel calm
• siento by itself usually means I feel something, as in perceiving or regretting something:
  • Siento frío = I feel cold
  • Siento mucho la noticia = I’m very sorry to hear the news

So in this sentence, me siento is the correct verb form because it describes the speaker’s state or condition.

Because tranquilo is an adjective, and after sentirse Spanish uses an adjective to describe how someone feels.

• me siento tranquilo = I feel calm
• tranquilamente means calmly, which is an adverb and would describe how an action is done, not a person’s state

So:

• Habla tranquilamente = He/she speaks calmly
• Se siente tranquilo = He/she feels calm

Más means more.

So:

• tranquilo = calm
• más tranquilo = calmer / more calm

The sentence means the speaker feels calmer than before, probably because of the run.

If you removed más, the meaning would change slightly:

• me siento tranquilo = I feel calm
• me siento más tranquilo = I feel calmer

It depends on the gender of the speaker.

• A male speaker would usually say tranquilo
• A female speaker would usually say tranquila

So if the speaker is female, the sentence would be:

• Después de haber corrido en la playa, me siento más tranquila.

The adjective agrees with the person who is feeling that way.

Because en la playa means on the beach or at the beach, which describes where the running happened.

• correr en la playa = to run on the beach

By contrast, a la playa usually indicates movement toward the beach:

• Voy a la playa = I’m going to the beach

So in this sentence, the speaker is not talking about going to the beach, but about running there, so en la playa is the natural choice.

Because the sentence describes a present result of a past action.

• haber corrido shows the earlier action: running
• me siento shows the current state: I feel calmer now

This is very natural in both Spanish and English:

• After running, I feel better
• Después de correr, me siento mejor

The first action is in the past relative to the second, but the main feeling is happening now.

It is generally recommended here because the sentence begins with a long introductory phrase:

• Después de haber corrido en la playa, me siento más tranquilo.

That comma helps separate the introductory time expression from the main clause.

In short:

• With a longer opening phrase, the comma is natural and helpful.
• In very short phrases, Spanish sometimes omits it.

So in this sentence, the comma is a good choice.

Yes. A common alternative is:

• Después de que corrí en la playa, me siento más tranquilo.

That means roughly After I ran on the beach, I feel calmer.

However, después de + infinitive is very common when the subject is the same in both parts of the sentence. Since I am the one who ran and also the one who feels calmer, the infinitive structure is especially natural:

• Después de correr...
• Después de haber corrido...

So the original sentence is elegant and very idiomatic.