Learn Hardcore Croatian: Dok duboko dišemo u parku, osjećamo da nam se pluća čiste i da je taj trenutak koristan za zdravlje. - While we breathe deeply in the park, we feel that our lungs are being cleaned and that this moment is useful for our health.

a specific moment: Kad sam došao, on je otišao. – When I came, he left.
or a general condition: Kad dišemo duboko, osjećamo se bolje. – When(ever) we breathe deeply, we feel better.

Here the idea is: during the very time that we are breathing deeply, so dok is more natural than kad.
You could say Kad duboko dišemo u parku..., but it would sound more like a general rule, less like “in this very moment while we are doing it”.

Why is it dok duboko dišemo and not dok dišemo duboko? Where do adverbs usually go?

Both dok duboko dišemo and dok dišemo duboko are grammatically correct.

In Croatian, adverbs like duboko (deeply) can usually go:

before the verb: duboko dišemo
or after the verb: dišemo duboko

The difference is usually very small, often just rhythm or emphasis:

duboko dišemo slightly emphasizes how we are breathing as part of the action,
dišemo duboko can sound a bit more like you first state the action (we breathe) and then comment on the manner (deeply).

In everyday speech, duboko dišemo (adverb before the verb) is more common and sounds smoother here.

Why is it u parku and not u park? What case is used with u?

U parku is locative case and is used for location (where something is).

With u, Croatian distinguishes:

u + accusative = movement into:
- Idemo u park. – We are going into the park.
u + locative = location (no movement):
- Dišemo u parku. – We are breathing in the park.

In your sentence there is no movement; it’s describing where the breathing happens.
So u parku (locative) is correct, not u park (accusative).

Why is it osjećamo da... and not something like osjećamo naša pluća čistiti se with an infinitive?

Croatian normally uses da + a finite verb (a full conjugated verb) after verbs like:

osjećati (to feel)
misliti (to think)
znati (to know)
nadati se (to hope)
željeti (to want), etc.

So:

osjećamo da nam se pluća čiste = we feel that our lungs are getting clean.
mislim da je to dobro = I think that it is good.
znam da on radi = I know that he is working.

A construction like osjećamo naša pluća čistiti se is not natural Croatian.
Where English can use infinitives (to do / doing), Croatian very often prefers a da + present tense clause.

What is the role of nam in osjećamo da nam se pluća čiste? Could we leave it out?

Nam is a dative pronoun meaning to us / for us.
Literally: osjećamo da nam se pluća čiste ≈ we feel that the lungs are getting clean to us / for us.

This is called dative of interest or dative of possession:

Boli me glava. – My head hurts. (literally: the head hurts me)
Operali su mu auto. – They washed his car. (literally: they washed to him the car)
Čiste nam se pluća. – Our lungs are getting clean (to us).

If you say osjećamo da se pluća čiste, it is still correct and understandable.
Adding nam:

makes it clearer it’s our lungs,
adds a more personal, “it’s happening to us” feeling.

So nam is not absolutely required, but it is natural and expressive here.

What does se do in pluća se čiste? Why not just pluća čiste?

Se marks a reflexive or middle construction.

čistiti nešto = to clean something
- Čistimo park. – We are cleaning the park. (someone actively cleans an object)
čistiti se = to get cleaned / to become clean (no explicit cleaner)
- Pluća se čiste. – The lungs are being cleaned / are cleansing themselves.

Without se, pluća čiste would mean the lungs clean (something), and you would expect an object:

Pluća čiste zrak. – The lungs clean the air.

In your sentence, the focus is on the process happening to the lungs, not on them cleaning something else, so pluća se čiste is the right form.

Why is the order da nam se pluća čiste and not da se nam pluća čiste or da pluća se čiste nam?

Croatian has a strict order for clitics (short unstressed words like se, mi, ti, mu, nam, ga, je, ih, etc.) and they prefer to stand in “second position” in a clause.

In da nam se pluća čiste:

da starts the clause,
then comes the clitic group nam se,
then the subject pluća, then the verb čiste.

The standard order inside the clitic group is roughly:

auxiliary verbs (sam, si, je, bih, bi…)
dative pronouns (mi, ti, mu, joj, nam, vam, im)
accusative/genitive pronouns (me, te, ga, je, ih…)
se (reflexive)

So nam se is correct, and se nam would be wrong in standard language.

You also can’t push the clitics to the end:

da pluća se čiste nam is ungrammatical.

So the normal, grammatical order is da nam se pluća čiste.

Why is pluća plural? Is it grammatically plural or just meaning a pair?

Pluća is grammatically neuter plural and it means lungs (the pair of lungs).

It is one of several body-part nouns that exist only in the plural (so‑called pluralia tantum):

pluća – lungs
leđa – back
usta – mouth (literally: mouths)
prsa – chest

So you say:

Pluća su zdrava. – The lungs are healthy. (verb in plural)
Pluća se čiste. – The lungs are getting clean.

There is a singular form plućе, but it is used only in special, mostly medical or anatomical contexts (one lung). In everyday language you just use pluća as plural.

Why is it taj trenutak and not ovaj trenutak or to vrijeme? What does taj express?

Croatian has three main demonstratives:

ovaj – this (near the speaker)
taj – that (near the listener or already known / just mentioned)
onaj – that (far from both, more distant in space or time)

In taj trenutak, taj often refers to:

something already introduced or defined by context,
or the specific situation being talked about, a “that moment we’re discussing”.

Here, the whole situation “while we breathe deeply in the park” defines taj trenutak.
You could say:

ovaj trenutak – more like this very moment (right now), slightly more immediate,
taj trenutak – that moment (that we’re talking about), a bit more neutral and typical in such general descriptions.

To vrijeme would mean that time / that period, more extended than a single moment.
Trenutak is a moment, a short point in time, which fits the idea better.

Why is it koristan za zdravlje and not something like koristan zdravlju or koristan za zdravlja?

The normal expression is koristan za + accusative, so:

koristan za zdravlje – beneficial for health
korisno za djecu – useful for children
dobro za kožu – good for the skin

The preposition za almost always takes the accusative case, not dative or genitive.
So:

zdravlje (accusative = same as nominative for neuter singular) is correct,
zdravlju (dative/locative) after za would be wrong,
zdravlja (genitive) after za would also be wrong.

Koristan zdravlju without za would sound unusual; with adjectives like koristan, the idiomatic pattern is koristan za + accusative.