Learn Hardcore Croatian: Doktor kaže da bi trebao duboko disati kad osjećaš stres. - The doctor says that you should breathe deeply when you feel stress.

In Croatian you normally cannot drop da in this kind of sentence – you need it to link the main clause (Doktor kaže) with the subordinate clause (da bi trebao…).

What exactly does da bi trebao mean? Why not just da trebaš?

Both are possible, but they are not identical:

da bi trebao duboko disati
– uses the conditional. This sounds like “you should”, “it would be good if you…”. It’s often softer, more advisory, less direct.
da trebaš duboko disati
– uses the present tense of trebati. This is more like “you need to / you must”, a bit stronger or more matter‑of‑fact.

So:

Doktor kaže da bi trebao duboko disati…
→ The doctor says (that) you should breathe deeply…
Doktor kaže da trebaš duboko disati…
→ The doctor says (that) you need to / must breathe deeply…

Why is it trebao (masculine) and not trebala or trebalo?

Trebao is the l‑participle (the form used in past tense and conditional) of trebati, and in Croatian it agrees with the subject in gender and number.

The implied subject here is ti (you singular). So:

If you are male:
ti bi trebao duboko disati
If you are female:
ti bi trebala duboko disati

In neutral or general advice (e.g. in a textbook example), you often see the masculine trebao, but in real life you change it based on who you’re talking to.

Plural examples:

to one man + one woman: vi biste trebali duboko disati
group of only women (colloquial but common): vi biste trebale duboko disati

Why is the verb after trebao in the infinitive (disati) and not in a personal form (dišeš)?

With trebati in this meaning (to need / should), the standard Croatian structure is:

trebati + infinitive

So:

trebao duboko disati = should breathe deeply
not
✗ trebao duboko dišeš

Compare:

Moram učiti. – I have to study.
Želim spavati. – I want to sleep.
Trebao bi odmoriti se. – You should rest.

Using da + present (trebaš da dišeš duboko) is Serbian or non‑standard in Croatian. Standard Croatian prefers trebati + infinitive.

Can I change the word order in trebao duboko disati or duboko disati? For example, trebao disati duboko?

Yes, you can:

trebao duboko disati
trebao disati duboko

Both are grammatically correct. The difference is mostly about rhythm and slight emphasis:

duboko disati puts a bit more focus on the type of action (deep breathing as an activity).
disati duboko can sound like you’re first mentioning the action (breathe), then specifying how (deeply).

In everyday speech they are practically interchangeable here.
What you cannot do is split trebao from its infinitive too much or put trebao at the end:

✗ duboko disati trebao – ungrammatical.

Is there any difference between kad and kada in kad osjećaš stres?

Kad and kada mean the same thing: when.

kad osjećaš stres
kada osjećaš stres

Both are correct. Differences:

kad – shorter, more colloquial, very common in speech and informal writing.
kada – slightly more formal or careful, often preferred in written or official styles.

You can use them interchangeably in this sentence.

Why is it osjećaš stres and not osjećaš se stresno or kad si pod stresom? Which is more natural?

All of these are possible, but they differ slightly in style and nuance:

kad osjećaš stres
– literal: when you feel stress
– correct, but in everyday speech it’s a bit less common than the other options.
kad si pod stresom
– when you are under stress / when you’re stressed
– very natural and frequent in everyday Croatian.
kad se osjećaš pod stresom
– when you feel (yourself) under stress
– also natural, slightly more “psychological” or introspective.
kad se osjećaš stresno
– understandable, but less idiomatic; Croatians more commonly say pod stresom rather than using stresno about a person.

So in spontaneous speech, you’ll more often hear:

Doktor kaže da bi trebao duboko disati kad si pod stresom.
…kad se osjećaš pod stresom.

What’s the difference between osjećaš and osjetiš? Could I say kad osjetiš stres?

Osjećati (imperfective) vs osjetiti (perfective):

osjećaš (from osjećati)
– ongoing or repeated feeling
– you feel / you are feeling
osjetiš (from osjetiti)
– moment when you first notice or experience something
– you feel / you notice (at a specific moment)

In your sentence:

kad osjećaš stres – while you are under stress; a state.
kad osjetiš stres – the moment when you start to feel stress.

The original sentence wants a general instruction about managing stress whenever you’re in that state, so osjećaš is more natural here.
Kad osjetiš stres is also possible, but it shifts the focus slightly to the initial moment when you detect stress.

Do I need a comma before kad in …duboko disati kad osjećaš stres?

No comma is needed here in standard Croatian:

Doktor kaže da bi trebao duboko disati kad osjećaš stres.

Kad osjećaš stres is an adverbial clause of time tightly connected to disati. In Croatian, these “when/if/ because”-type clauses usually do not take a comma when they follow the main clause and are short and integrated.

You might see a comma in some contexts for emphasis or in long, complex sentences, but here the natural, standard version is without a comma.

Why is it Doktor kaže (present) and not Doktor je rekao (past) in Croatian? Is the English equivalent also present?

Both tenses are possible in Croatian; it depends on what you want to express:

Doktor kaže… – present
→ The doctor says…
– can mean something the doctor generally says (habitual advice)
– or something the doctor is saying now or regularly.
Doktor je rekao… – past (perfect)
→ The doctor said…
– refers to a specific past occasion when the doctor said it.

In your sentence, using kaže suggests general advice or something you accept as currently valid. English would most naturally mirror that:

Doktor kaže da bi trebao…
→ The doctor says (that) you should…

If you told a story about a past visit, you’d more likely say:

Na pregledu mi je doktor rekao da bih trebao duboko disati kad osjetim stres.
→ At the check‑up, the doctor told me I should breathe deeply when I feel stress.

Can I use liječnik instead of doktor?

Yes:

Doktor kaže…
Liječnik kaže…

Both mean doctor, but there is a slight nuance:

doktor
– very common in everyday speech for a medical doctor
– also used for people with a doctoral degree (PhD), but context clarifies.
liječnik
– literally physician, medical doctor
– a bit more formal; used in official language, documents, the healthcare system.

In normal conversation about a medical doctor, doktor is probably more frequent, but liječnik is equally correct.

Why is there no ti (you) in kad osjećaš stres or da bi trebao…? Is it optional?

Croatian is a pro‑drop language: subject pronouns are usually omitted because the verb ending already shows who the subject is.

osjećaš → clearly ti (you singular)
bi trebao in context also refers to ti

You can add ti for emphasis or contrast:

Doktor kaže da bi ti trebao duboko disati kad osjećaš stres.
→ The doctor says that *you (as opposed to someone else) should breathe deeply when you feel stress.*

But in neutral, non‑emphatic sentences, it’s more natural to leave out ti.

What does bi itself mean in da bi trebao? Is it like would in English?

Bi is the conditional auxiliary in Croatian, comparable to English would, but its translation depends on the main verb:

bi + l‑participle = conditional mood

Examples:

Ja bih došao. – I would come.
Ti bi trebao duboko disati. – literally You would need / would have to breathe deeply.

In real translation:

ti bi trebao… → you should…
ja bih trebao… → I should…

So:

bi on its own corresponds to would
bi trebao as a phrase is best translated as should in this context.