Learn Hardcore Croatian: Kad duboko dišem, lakše spavam. - When I breathe deeply, I sleep more easily.

Kad – subordinating conjunction meaning when.
duboko – adverb meaning deeply, modifying the verb dišem.
dišem – 1st person singular, present tense of disati (to breathe): I breathe.
lakše – comparative adverb meaning more easily / more lightly, modifying spavam.
spavam – 1st person singular, present tense of spavati (to sleep): I sleep.

So structurally you have:

Kad duboko dišem – a time/condition clause (when I breathe deeply),
lakše spavam – the main clause (I sleep more easily).

What is the difference between kad and kada? Can I use both here?

kad and kada mean the same: when.
kada is the full form; kad is the shorter, more colloquial form.

In most everyday contexts they are completely interchangeable:

Kad duboko dišem, lakše spavam.
Kada duboko dišem, lakše spavam.

Kada is slightly more formal or emphatic and is found more often in careful writing, but both are correct.

Why is the present tense used here if the meaning is more general (like a habit)?

In Croatian, the present tense is used very often for:

general truths and
habits / repeated actions,

just like in English.

So:

Kad duboko dišem, lakše spavam.
literally: When I breathe deeply, I sleep more easily.
This describes a general relationship, not one single event.

If you wanted to refer to the past, you would change both verbs:

Kad sam duboko disao, lakše sam spavao. – When I breathed deeply, I slept more easily.

Why is there no “ja” in the sentence? Can I say “Ja kad duboko dišem, lakše spavam”?

Croatian usually omits subject pronouns (ja, ti, on, ona, …) because the verb ending already shows the person.

dišem and spavam clearly show 1st person singular (I).
So adding “ja” is not necessary in neutral sentences.

You can use ja for emphasis, but you must place it carefully:

Kad ja duboko dišem, lakše spavam.
= When I breathe deeply, I sleep more easily (as opposed to someone else).

Putting ja at the very beginning (Ja kad duboko dišem…) sounds clumsy and is not typical.

What exactly is duboko grammatically? Is it an adjective or an adverb?

Duboko comes from the adjective dubok (deep).

As an adjective, you’d see forms like dubok, duboka, duboko (deep).
As an adverb, you normally use the -o form: duboko = deeply.

In Kad duboko dišem, duboko is an adverb modifying the verb dišem:

duboko dišem – I breathe deeply.

The same pattern appears in many adjectives:

brz → brzo – fast → quickly
tiha → tiho – quiet → quietly

What exactly is lakše? Is it “easier”, “lighter”, or “better”?

The base adjective is lak:

lak = light (in weight) / easy.
Its basic adverb form is lako = easily.

The comparative forms are:

adjective: lakši = lighter / easier (as an adjective),
adverb: lakše = more easily / more lightly.

In lakše spavam, lakše is an adverb:

lakše spavam = I sleep more easily (it is easier for me to sleep / fall asleep).

Depending on context, it can feel close to “I sleep more easily / with less difficulty”, sometimes overlapping with “I sleep better”.

Why is there a comma after Kad duboko dišem?

In Croatian, when a subordinate clause (introduced by kad, ako, jer, iako, etc.) comes before the main clause, it is normally followed by a comma.

So:

Kad duboko dišem, lakše spavam. – correct punctuation.

If you reverse the order, a comma is usually not used in simple sentences:

Lakše spavam kad duboko dišem. – also correct; usually written without a comma.

Can I change the word order? For example: Kad dišem duboko, lakše spavam or Lakše spavam kad duboko dišem?

Yes, Croatian word order is quite flexible, and all of these are grammatically fine:

Kad duboko dišem, lakše spavam.
Kad dišem duboko, lakše spavam.
Lakše spavam kad duboko dišem.
Spavam lakše kad duboko dišem.

Differences are mostly about rhythm and emphasis:

Putting lakše right before spavam (lakše spavam) slightly emphasizes how you sleep.
Starting with Lakše spavam… can emphasize the result: The important point is that I sleep more easily, and the condition is when I breathe deeply.
Moving duboko before or after dišem doesn’t change the meaning much here.

What is the difference between kad and dok? Could I say Dok duboko dišem, lakše spavam?

kad = when, can mean whenever or mark a time/condition.
dok = while, emphasizes simultaneity (two things happening at the same time).

In your sentence, kad is better because it implies a condition or repeated situation:

Kad duboko dišem, lakše spavam.
= Whenever I breathe deeply, I sleep more easily.

Dok duboko dišem, lakše spavam sounds more like:

While I am breathing deeply, I sleep more easily,
which is odd, because you are not normally actively focusing on deep breathing during sleep. So dok is not natural here.

Could I say Bolje spavam instead of Lakše spavam? Is there a difference?

You can say both, but the nuance changes:

lakše spavam (from lako)
= I sleep more easily / It’s easier for me to fall asleep / stay asleep.
Focus: less difficulty in sleeping.
bolje spavam (from dobro → bolje)
= I sleep better.
Focus: better quality of sleep (deeper, more restful, etc.).

In many contexts they overlap and both would be understood as an improvement in sleep, but:

lakše spavam emphasizes ease,
bolje spavam emphasizes quality.

Can I say Kad duboko udahnem, lakše spavam? How is that different from Kad duboko dišem?

Yes, you can, but the meaning changes slightly.

dišem (imperfective aspect) = I breathe (ongoing or habitual action).
- Kad duboko dišem, lakše spavam.
  = When(ever) I breathe deeply (as a habit / in general), I sleep more easily.
udahnem (perfective aspect, from udah(n)uti/udahnem) = I take a breath / I inhale once.
- Kad duboko udahnem, lakše spavam.
  = When I take a deep breath, I sleep more easily. (suggests a specific act or a few deep breaths done before sleep).

So:

dišem → focuses on the ongoing way of breathing,
udahnem → focuses on a single deep inhale.

Could I use ako instead of kad, like Ako duboko dišem, lakše spavam? What is the difference?

ako = if (conditional),
kad = when / whenever (time, often with a habitual/conditional flavor).

Compare:

Kad duboko dišem, lakše spavam.
= Whenever I breathe deeply, I sleep more easily.
This sounds like a regular pattern or general rule.
Ako duboko dišem, lakše spavam.
= If I breathe deeply, I sleep more easily.
This sounds a bit more hypothetical, like you are talking about a condition that may or may not be fulfilled.

Both are understandable, but kad is more natural for describing a known, repeated situation.