Learn Hardcore Croatian: Kad imam jak kašalj, ostajem u krevetu i budan slušam tihi zvuk kiše. - When I have a strong cough, I stay in bed and awake I listen to the quiet sound of the rain.

kad = shorter, more colloquial, very common in speech and in informal writing.
kada = slightly more formal or emphatic, often seen in writing, but also used in speech.

In this sentence, you could say Kad imam jak kašalj… or Kada imam jak kašalj… with no change in meaning.

Why is the verb in the present tense: imam, ostajem, slušam, and not some future form?

The present tense here expresses a general, habitual situation: what you usually do whenever this condition happens.

Kad imam jak kašalj, ostajem u krevetu… = Whenever I have a bad cough, I (normally) stay in bed…

This is the same use of the present as in English:
“When I have a bad cough, I stay in bed and listen…”

So the Croatian present matches the English present for this “whenever X, I (normally) do Y” meaning.

What is the structure of imam jak kašalj? Why is it not something like “I am with a strong cough”?

Croatian uses imati + noun much more than English uses “have” this way.

imam = I have
jak = strong (masculine singular adjective)
kašalj = cough (masculine noun)

So imam jak kašalj is literally “I have a strong cough”.

Grammar details:

kašalj is the direct object of imam, so it is in the accusative case.
For masculine animate/inanimate in the singular, nominative and accusative often look the same, so you just see kašalj.
jak agrees with kašalj in gender (masc.), number (sing.), and case (acc.), and here that form is also jak.

No article is needed (Croatian has no articles), so imam jak kašalj corresponds to “I have a strong cough” (or “a bad cough”) in English.

Why jak kašalj and not jaki kašalj?

Both jak and jaki are forms of the adjective jak (strong), but they are used in different contexts:

jak – masculine singular nominative/accusative in the indefinite form.
jaki – could be masculine plural, or masculine singular definite form (roughly, “the strong…”).

In a simple, indefinite phrase like “a strong cough” as a direct object, you use jak kašalj.

Jaki kašalj could appear in contexts where you are talking about a specific, known cough (“that strong cough”), or in plural (“strong coughs”), but not in this simple generic statement.

Why is it ostajem and not ostanem?

Croatian verbs come in aspect pairs. Here:

ostajati (impf.) – ostajem = to be staying, to stay (habitually / over a period of time)
ostati (pf.) – ostanem = to stay (once, as a completed event)

In your sentence, you’re describing what you usually do whenever you have a cough, so the imperfective aspect (ostajem) is natural for habitual, repeated actions:

Kad imam jak kašalj, ostajem u krevetu…
= Whenever I have a bad cough, I (usually) stay in bed…

If you were talking about a single event, you’d be more likely to use ostanem or a future form:

Kad dobijem jak kašalj, ostat ću u krevetu.
When I get a bad cough, I will stay in bed.

Why u krevetu, not u krevet?

Both are correct Croatian, but they mean different things:

u krevetu = in bed (locative case)
→ You are already in bed, staying there, remaining lying there.
u krevet = into bed (accusative case)
→ Movement into the bed: “I go/get into bed.”

In this sentence, the meaning is:
“I stay in bed” → ostajem u krevetu, so the locative with u is used.

What exactly does budan do in i budan slušam? Why not budno?

Budan is an adjective meaning “awake” (masculine singular). It stands in the nominative and describes the subject “I”:

Literally: “…and (being) awake I listen to…”

So the structure is:

(Ja) budan slušam… = I, awake, listen…

Croatian often uses an adjective like this as a kind of secondary predicate describing the subject during the action.

Budno would be an adverb (“in an awake/attentive way”), slightly different in meaning.
Budan says that you are in the state of being awake; budno would describe how you are listening (attentively, watchfully).

Could I also say slušam budan tihi zvuk kiše? Is the word order important?

Yes, Slušam budan tihi zvuk kiše is also correct.

Croatian word order is relatively flexible. Both:

budan slušam tihi zvuk kiše
slušam budan tihi zvuk kiše

are understandable and grammatical.

Subtle differences:

budan slušam…: puts budan in a bit more prominent position; almost “awake, I listen…”
slušam budan…: slightly more neutral; “I listen, (being) awake…”

But in everyday speech, both would usually be taken the same way.

What cases are used in tihi zvuk kiše, and why?

Let’s break it down:

slušam – I listen (to) → takes a direct object in the accusative.
tihi zvuk – quiet sound
- zvuk is the direct object → accusative singular masculine.
- tihi is an adjective agreeing with zvuk → masculine singular accusative (definite form).
kiše – of rain
- genitive singular of kiša (rain), used to show possession/origin (“sound of rain”).

So the structure is literally:
slušam tihi zvuk kiše = I listen to the quiet sound of the rain.

Accusative: tihi zvuk (what you listen to)
Genitive: kiše (whose/what sound it is).

Why is it zvuk kiše instead of just slušam kišu?

Both are possible, but they focus on different things:

slušam kišu – I listen to the rain
→ Object is the rain itself.
slušam tihi zvuk kiše – I listen to the quiet sound of the rain
→ Object is the sound, specifically the sound produced by the rain.

The sentence you have highlights the sound (zvuk) and describes it as tihi (quiet), giving a more detailed, almost poetic image.

Why is kiše and not kiša used here?

Kiša is the nominative form (rain as the subject). Here, kiše is in the genitive case:

zvuk kiše – the sound of rain

In Croatian, when you express “X of Y” (sound of rain, smell of coffee, color of the sky, etc.), Y is usually in the genitive:

miris kave – smell of coffee
boja neba – color of the sky
zvuk kiše – sound of rain

So kiše (genitive singular) is required by that “X of Y” relationship.

Why is there a comma after kašalj: Kad imam jak kašalj, ostajem u krevetu…?

Croatian normally uses a comma between a subordinate clause and the main clause.

Here:

Kad imam jak kašalj – subordinate clause introduced by kad.
ostajem u krevetu i budan slušam tihi zvuk kiše – main clause.

So you separate them with a comma:

Kad imam jak kašalj, ostajem u krevetu…

How do you pronounce kašalj and kiše, especially š and lj?

š = like English “sh” in “ship”.
lj = a palatal sound, similar to the “ll” in some pronunciations of “million” ([mi-lyən]).

So roughly:

kašalj ≈ KAH-shal(y)
- ka as in “car” (shorter)
- š as in “ship”
- lj = [ly] sound at the end.
kiše ≈ KEE-she
- ki as in “key”
- še with š like “sh” and e like “bet”.

Both š and lj are single letters in Croatian and always have these sounds.