Learn Hardcore Croatian: Kad duboko dišem, osjećam se manje umorno. - When I breathe deeply, I feel less tired.

They mean the same. Kad is just a shorter, more colloquial form; kada can sound a bit more formal or emphatic, but both are completely correct and very common.

Does Kad duboko dišem mean “when I breathe deeply (right now)” or “whenever I breathe deeply”?

With kad(a) + present tense in Croatian, the meaning is usually general / habitual:

Kad duboko dišem, osjećam se manje umorno.
→ “Whenever I breathe deeply, I feel less tired.”

Context can also make it refer to specific future situations:

Kad dođem, nazvat ću te. – “When I come, I’ll call you.”

In your sentence, it’s most natural to understand it as “whenever I breathe deeply”, i.e. as a general rule.

Why is there a comma after Kad duboko dišem?

Croatian normally puts a comma between a subordinate clause and the main clause, regardless of the order.

So when the kad-clause comes first, you write:

Kad duboko dišem, osjećam se manje umorno.

If you reverse the order, you still usually write a comma:

Osjećam se manje umorno, kad duboko dišem.

In practice the comma after the main clause is sometimes dropped in informal writing, but the version with a comma is what grammars recommend.

Why is it duboko dišem and not dišem duboko? Are both correct?

Duboko is an adverb (“deeply”) modifying dišem (“I breathe”).

Both orders are grammatically fine:

Kad duboko dišem, …
Kad dišem duboko, …

The basic neutral order is verb + adverb (dišem duboko), but putting the adverb in front (duboko dišem) puts a bit more emphasis on how you breathe: it sounds slightly more contrastive, like “when I breathe deeply (as opposed to shallowly)”. In everyday speech you will hear both variants.

Why do we need se in osjećam se? Why not just osjećam manje umorno?

The verb osjećati means “to feel / sense something” and normally takes an object:

Osjećam umor. – “I feel tiredness.”
Osjećam bol. – “I feel pain.”

When you want to say “I feel (in some way)” about your own state, Croatian usually uses the reflexive form osjećati se:

Osjećam se umorno. – “I feel tired.”
Osjećam se sretno. – “I feel happy.”

So in your sentence, se is needed to express “I feel (myself) less tired”, not “I feel less tiredness as an object”. Without se, you’d expect a noun after osjećam, not an adjective/adverb.

Where exactly does se have to go in the sentence? Could I say se osjećam manje umorno or osjećam manje se umorno?

Se is an unstressed clitic and in Croatian clitics must appear in second position in their clause (after the first stressed word/phrase).

In your main clause, the first stressed word is osjećam, so the normal order is:

osjećam se manje umorno

Some other correct possibilities, showing the “second position” rule:

Ja se osjećam manje umorno. (first word = Ja, so se comes second)
Manje se umorno osjećam. (first word = Manje, so se comes second)

These are wrong:

*se osjećam manje umorno (starts with a clitic – not allowed)
*osjećam manje se umorno (clitic not in second position)

Why is it umorno and not umoran or umorna? Doesn’t it need to match my gender?

The adjective umoran has these forms:

masculine: umoran
feminine: umorna
neuter: umorno

But in Croatian, the neuter form is also used as an adverb, describing how something is done, not matching the subject’s gender. In your sentence:

umorno is functioning as an adverb of manner: “I feel tiredly / in a tired way.”

That’s why it does not change with the speaker’s gender here; both a man and a woman will naturally say:

Osjećam se umorno. – “I feel tired.”

So there’s no mistake: umorno is adverbial here, not a gendered adjective.

Can I say osjećam se manje umoran or manje umorna instead of manje umorno?

Yes, these are also correct and very natural:

(male speaker) Osjećam se manje umoran.
(female speaker) Osjećam se manje umorna.

Here manje (“less”) modifies the adjective (umoran/umorna), so the form does agree with your gender.

The difference is subtle:

manje umoran / manje umorna → highlights your state as a property of you (adjective).
manje umorno → sounds a bit more like describing how you feel (adverbial), slightly more neutral/general.

In everyday speech, Osjećam se manje umorno works for everyone and is very common.

What exactly does manje do in manje umorno?

Manje is the comparative form of malo (“a little”), and here it works like English “less”.

In manje umorno it is an adverb of degree that modifies umorno:

umorno – “tired(ly)”
manje umorno – “less tired(ly)”

You can use manje the same way with many adjectives/adverbs:

manje gladan – less hungry
manje sretan – less happy
manje brzo – less quickly / more slowly (literally “less fast”)

Can I change the word order, for example: Osjećam se manje umorno kad duboko dišem?

Yes. Word order in Croatian is fairly flexible, especially for adverbial clauses like this. All of these are fine (with a comma usually preferred):

Kad duboko dišem, osjećam se manje umorno.
Osjećam se manje umorno, kad duboko dišem.
Osjećam se manje umorno kad duboko dišem. (comma often omitted in casual writing)

Moving parts around usually changes focus / emphasis, not the basic meaning. Starting with the kad-clause makes the condition (“when I breathe deeply”) more prominent; starting with Osjećam se… emphasizes your feeling first.

What’s the difference between Kad duboko dišem and Kad duboko udahnem?

dišem = “I breathe” in general (ongoing / habitual), from disati (imperfective)
udahnem = “I take a breath / inhale once”, from udah(n)uti (perfective)

So:

Kad duboko dišem, osjećam se manje umorno.
→ “When(ever) I breathe deeply, I feel less tired.” (general habit)
Kad duboko udahnem, osjećam se manje umorno.
→ “When I take a deep breath, I feel less tired.” (single deep breath each time)

Both are correct; choose dišem for continuous or habitual breathing, udahnem for a single deep inhale.

How are š and ć pronounced in dišem and osjećam?

š is like “sh” in “shoe”:
- dišem ≈ DEE-shem
ć is a soft “ch”, a bit like the “t” in British English “tune” when pronounced tyoon, or a soft version of “ch” in “chew”:
- osjećam ≈ O-sye-cham (with a very soft, palatal “ch”)

Croatian consistently pronounces these letters the same way in all words, so once you learn them, you can apply them everywhere.