Trčanje u parku mi pomaže da se osjećam opuštenije.

It is a noun, even though it comes from the verb trčati (to run).

• trčati = to run
• trčanje = running

This kind of form is called a verbal noun. In this sentence, trčanje u parku works as the subject of the sentence, just like running in the park does in English.

It is grammatically neuter singular, which is why the verb is also singular: pomaže.

Because the subject is singular:

• Trčanje u parku = running in the park

Even though the idea may feel broad in English, Croatian treats trčanje as one singular noun, so the verb must also be singular:

• trčanje ... pomaže

If the subject were plural, then you would use pomažu.

Because u is the normal preposition for being in a park.

Here the meaning is location, not movement, so u takes the locative case:

• park → u parku

A useful contrast:

• u park = into the park, motion toward it
• u parku = in the park, location

So in this sentence, u parku means in the park.

Normally, no. U parku is the natural Croatian expression.

Croatian uses u with places like parks, cities, rooms, buildings, and enclosed areas.
The preposition na is more often used for:

• surfaces: na stolu = on the table
• some public places/institutions/events: na poslu, na koncertu, na fakultetu

A park is normally understood as a place you are in, so u parku is the correct choice.

Mi means to me.

It is the dative singular clitic form of ja. The verb pomagati / pomoći usually takes the person being helped in the dative:

• To mi pomaže. = That helps me.
• Trčanje u parku mi pomaže... = Running in the park helps me...

So mi is the person affected by the helping.

Because the verb requires the dative case, not nominative or accusative.

• ja = I, subject form
• mene = me, usually accusative/genitive
• mi = to me, dative

Since pomaže means helps, Croatian expresses that as helps to me, so the correct form is mi.

If you want emphasis, you can also use meni:

• Trčanje u parku meni pomaže...

But in a neutral sentence, mi is the normal choice.

Croatian word order is fairly flexible, but mi is a clitic, and clitics usually go in the second position of the clause.

That is why this sounds natural:

• Trčanje u parku mi pomaže...

The first unit is Trčanje u parku, and the clitic mi comes right after that unit.

You may hear other orders in real speech, but this version is very natural and standard. The important thing is that clitics like mi, se, sam, je, and so on usually cannot stand freely wherever you want.

Because Croatian very often uses da + present tense where English uses to + verb.

So:

• pomaže mi da se osjećam... = helps me to feel...

This is one of the most common Croatian patterns. A literal infinitive-style translation is often not the most natural choice in Croatian.

You may sometimes see or hear infinitive-like structures in other contexts, but here da se osjećam is completely normal and natural.

Because the person who is feeling more relaxed is I, understood from mi.

So inside the da-clause, the hidden subject is:

• (ja) se osjećam = I feel

That is why the verb is in the first person singular present tense:

• osjećam

Even though ja is not written, Croatian often leaves subject pronouns out when the verb ending already shows the person.

Here se is part of the verb expression osjećati se, which means to feel.

Compare:

• osjećati = to sense, to feel something
• osjećati se = to feel, to feel a certain way

So in this sentence, se is necessary. Without it, the meaning changes.

For example:

• Osjećam hladnoću. = I feel the cold.
• Osjećam se umorno. = I feel tired.

So se osjećam means I feel in the sense of I feel a certain way.

Opuštenije means more relaxed. It is a comparative form.

The basic idea is:

• opušten / opušteno = relaxed
• opuštenije = more relaxed

After osjećati se, Croatian very often uses this kind of form naturally to describe someone’s state:

• Osjećam se bolje. = I feel better.
• Osjećam se opuštenije. = I feel more relaxed.

So the sentence is not saying simply relaxed, but specifically more relaxed.

Yes, you may hear those too, depending on the speaker’s gender:

• opušteniji = masculine
• opuštenija = feminine

Those forms agree more directly with the person speaking.

But opuštenije is also very common and natural in this kind of sentence, especially in everyday usage, where it functions in a more general more relaxed sense.

So for a learner, the important thing is:

• osjećam se opuštenije = perfectly natural
• osjećam se opušteniji / opuštenija = also possible, with clearer gender agreement

Not if you want to keep the meaning helps me.

Without mi, the sentence becomes less natural because pomaže usually needs to show who it helps:

• Trčanje u parku mi pomaže... = Running in the park helps me...
• without mi, that personal relationship is missing

So in this sentence, mi is an important part of the structure.

If you wanted a more general meaning, you would normally replace it with another dative phrase, for example:

• Trčanje u parku ljudima pomaže da se osjećaju opuštenije.
  = Running in the park helps people feel more relaxed.