Learn Hardcore Croatian: Kad počne oluja, čujemo jaku grmljavinu iznad grada. - When a storm starts, we hear loud thunder above the city.

kad and kada have the same meaning: “when”.
kada is the full form and sounds a bit more formal or careful.
kad is the shorter, more colloquial form you’ll hear all the time in speech.

You can say either:

Kad počne oluja, čujemo jaku grmljavinu.
Kada počne oluja, čujemo jaku grmljavinu.

Both are correct; the meaning is the same.

Why is it “Kad počne oluja” and not “Kad će početi oluja” for “When the storm starts”?

In Croatian, after time words like kad (when), čim (as soon as), dok (while), prije nego (before), you usually do not use future tense in the subordinate clause.

Instead, you often use:

Present tense of a perfective verb to talk about a future event.

So:

Kad počne oluja…
literally: When the storm begins…
but it can refer to the future: When the storm begins (later)…

“Kad će početi oluja” is grammatically possible but sounds unnatural here; it feels more like a direct question (When will the storm start?) than part of a complex sentence.

Natural patterns are:

Kad počne oluja, čut ćemo jaku grmljavinu.
When the storm starts, we will hear loud thunder.
Kad počne oluja, čujemo jaku grmljavinu.
Whenever a storm starts, we (usually) hear loud thunder. (habitual statement)

What is the difference between počne and počinje?

Both relate to starting/beginning, but they differ in aspect:

počne – from početi (perfective)
- one complete event, a point in time
- used when you focus on the moment something starts
- here: Kad počne oluja… = When the storm (actually) starts…
počinje – from počinjati (imperfective)
- ongoing, repeated, or habitual action
- used when you talk about how something usually starts, or that it is in the process of starting

Compare:

Kad počne oluja, idemo kući.
When the storm starts (at that moment), we go home.
Svaki put kad počinje oluja, postaje jako vjetrovito.
Every time a storm is starting, it becomes very windy.

In the original sentence, počne fits because we care about the moment the storm begins.

What grammatical role and form does oluja have in this sentence?

In Kad počne oluja, čujemo jaku grmljavinu iznad grada, the word oluja:

is a noun, feminine
is in nominative singular
functions as the subject of the verb počne

You could rewrite the clause as:

Oluja počne. – The storm starts.

So:

oluja = subject
počne = verb (3rd person singular, present, perfective)

Why is there a comma after “Kad počne oluja”?

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

Here:

Kad počne oluja – subordinate clause (time clause)
čujemo jaku grmljavinu iznad grada – main clause

Because the subordinate clause comes first, you must use a comma:

Kad počne oluja, čujemo jaku grmljavinu iznad grada.

If you reverse the order, a comma is usually optional and often omitted in modern usage:

Čujemo jaku grmljavinu iznad grada kad počne oluja.
(commonly written without a comma)

What does čujemo tell us about the subject, and could we add mi?

čujemo is:

verb čuti = to hear
1st person plural, present tense: we hear

Croatian verbs carry person and number information, so the subject pronoun is usually dropped:

Čujemo jaku grmljavinu. = We hear loud thunder.

You can add mi for emphasis:

Mi čujemo jaku grmljavinu. – We hear loud thunder (as opposed to someone else).

In the original sentence, leaving mi out is the most natural, neutral choice.

Why is it “jaku grmljavinu” and not “jaka grmljavina”?

Because “jaku grmljavinu” is the direct object of the verb čujemo, it must be in the accusative case.

Breakdown:

noun: grmljavina (thunder), feminine
- nominative singular: grmljavina
- accusative singular: grmljavinu
adjective: jak (strong, loud)
- nominative feminine singular: jaka
- accusative feminine singular: jaku

In the sentence:

čujemo (koga/što?) jaku grmljavinu
we hear (whom/what?) loud thunder → accusative

So:

jaka grmljavina – nominative (e.g. Jaka grmljavina se čuje. – Loud thunder is heard.)
jaku grmljavinu – accusative (here: čujemo jaku grmljavinu.)

What case is grada in, and why does iznad use that case?

grada is in the genitive singular of grad (city).

The preposition iznad = above / over always takes the genitive:

iznad (koga/čega?) grada – above the city

Other examples:

iznad kuće – above the house
iznad mora – above the sea
iznad parka – above the park

So the phrase:

iznad grada = above the city
and grada is genitive because of iznad.

Can the word order be changed, for example “Kad oluja počne, čujemo grmljavinu jaku iznad grada”?

Croatian word order is relatively flexible, but some orders sound more natural than others.

In the time clause, both are correct:
- Kad počne oluja…
- Kad oluja počne…
The second slightly emphasizes oluja (the storm), but both are normal.
In the main clause, the usual neutral order is:
- čujemo jaku grmljavinu iznad grada
- adjective jaku normally comes before the noun grmljavinu.
- iznad grada usually comes after the object.
Saying čujemo grmljavinu jaku iznad grada is grammatically possible but sounds poetic, marked, or emphatic, not neutral everyday speech.

So, you can adjust word order, but the original:

Kad počne oluja, čujemo jaku grmljavinu iznad grada.

is the most natural, standard version.

Is grmljavina the usual word for “thunder”? How is it different from grom?

Yes, grmljavina is a common word for thunder as a phenomenon (the thunder you hear in a storm).

grmljavina
- non-count, like “thunder” in English
- the general noise: thunder, rumbling
- e.g. Čujemo jaku grmljavinu. – We hear loud thunder.
grom
- literally: a (single) thunderbolt / clap of thunder
- often connected to the strike itself and also to the sound
- e.g. Čuo sam jedan jak grom. – I heard a loud thunderclap.

In your sentence, grmljavina fits well because it describes the continuous loud thunder over the city during the storm.