После прогулки в парке я успокаиваюсь.

The preposition после (after) always requires the genitive case.

• Nominative: прогулка (a walk)
• Genitive singular: прогулки

So after после you must say после прогулки = after (the) walk.
It is genitive singular, not plural. The ending -и here marks genitive singular for a feminine noun in -ка.

Formally, прогулки can be:

• genitive singular: of the walk
• nominative plural: walks

You understand which one it is from context and preposition:

• после always takes genitive → here прогулки = genitive singular.
• There is no reason for a plural here; the sentence is about a walk in the park as a type of situation, not a specific list of multiple walks.

So whenever you see после + прогулки, treat прогулки as genitive singular.

Because the meaning is during a walk *in the park (inside / within it), not *towards the park.

• в парк (+ accusative) = movement into the park
  
  • Я иду в парк. – I am going to the park.
• в парке (+ prepositional) = location inside the park
  
  • Я гуляю в парке. – I am walking in the park.

In после прогулки в парке, the idea is a walk that takes place in the park, so you need в парке (prepositional case of парк: парке).

Успокаиваюсь has the reflexive ending -сь, which corresponds roughly to English myself in I calm myself (down).

• успокаивать кого-то – to calm someone (else) down
  
  • Я успокаиваю ребёнка. – I am calming the child down.
• успокаиваться – to calm down (oneself, or to become calm)
  
  • Я успокаиваюсь. – I calm down / I become calm.

In this sentence you are talking about your own state after a walk, so you need the reflexive verb: я успокаиваюсь.

They are an aspect pair:

• успокаиваться – imperfective (process, repeated/habitual action)
  
  • Я успокаиваюсь после прогулки.
    I calm down after a walk (habitually, in general).
• успокоиться – perfective (single, completed result)
  
  • Я успокоюсь после прогулки.
    I will calm down after the walk (one specific time / result).

In your sentence, успокаиваюсь is used as a general or habitual statement about what usually happens after a walk, so the imperfective is correct.

It is present tense of the imperfective verb успокаиваться:

• я успокаиваюсь
• ты успокаиваешься
• он/она успокаивается
• мы успокаиваемся
• вы успокаиваетесь
• они успокаиваются

Here it describes either:

1. What generally happens (habitual present):
   After a walk in the park, I (usually) calm down.

2. What is happening now in a narrative sense:
   After the walk in the park, I am calming down (now).

It is not future; the future would use the perfective успокоиться → я успокоюсь.

You can, and it is grammatically correct, but it sounds more wordy and less natural than the original.

• После прогулки в парке я успокаиваюсь.
  – Compact and idiomatic: After a walk in the park, I calm down.

• После того как я гуляю в парке, я успокаиваюсь.
  – Literally: After the fact that I walk in the park, I calm down.
  Grammatically fine, but heavier and unnecessarily clumsy in everyday speech.

Native speakers prefer после + noun (in genitive) in this kind of sentence.

Yes, that is perfectly correct and very natural.

• После прогулки в парке я успокаиваюсь.
• Я успокаиваюсь после прогулки в парке.

Both mean the same. Russian word order is fairly flexible. Differences:

• Starting with После прогулки в парке slightly emphasizes the condition or time after which you calm down.
• Starting with Я slightly emphasizes you or your state.

But in neutral context, they are practically interchangeable.

No comma is needed.

It is a simple sentence with:

• a prepositional phrase (adverbial modifier): После прогулки в парке
• a subject: я
• a predicate (verb): успокаиваюсь

There are no separate clauses here, so there is no reason to put a comma.

Yes, you can say После прогулки по парку, and it is also correct, but the nuance changes slightly:

• прогулка в парке – a walk in the park, focusing on the location (inside the park).
• прогулка по парку – a walk around/through the park, focusing a bit more on movement within the park’s area.

In practice, both are often understood similarly, and in this sentence either is possible.
В парке is slightly more neutral; по парку adds a subtle sense of walking around the park’s paths/space.

Yes, but the meaning shifts:

• После прогулки в парке – after a walk in the park (one walk, or a walk as a general type of situation).
• После прогулок в парке – after walks in the park (repeated occasions / many walks).

Both can be general statements, but:

• Singular прогулки (genitive singular) sounds like “after a walk in the park, I (generally) calm down.”
• Plural прогулок (genitive plural) makes you think more about multiple walks as events that often happen.

Russian usually prefers reflexive verbs with -сь instead of explicit себя for this kind of meaning.

• я успокаиваюсь – I calm down / I calm myself (down).
• я успокаиваю себя – grammatically possible, but sounds unusual and overly analytical, used only in very specific stylistic contexts.

So the natural, everyday way to say I calm myself is simply я успокаиваюсь.

The stress is on the а:

успокАиваюсь → [u-spa-KA-i-va-yus’]

Syllable breakdown:

• у‑спо‑кА‑и‑ва‑юсь

Common learner mistake: putting stress later (e.g. успокаИваюсь). That is incorrect; keep it on кА.

You can say:

• После прогулки в парке я расслабляюсь.

The difference in nuance:

• успокаиваюсь – I calm down, become less anxious / less nervous, become emotionally and mentally calm.
• расслабляюсь – I relax, loosen up physically and mentally, stop being tense.

They overlap, but:

• If the focus is on reducing stress/anxiety → успокаиваюсь is better.
• If the focus is on relaxing your body/feeling laid-back → расслабляюсь fits more.