Learn Hardcore Croatian: Kad je dan sunčan, ne nosim kišobran. - When the day is sunny, I do not carry an umbrella.

kad = shorter, more colloquial, very common in speech and informal writing
kada = a bit more formal or careful style, common in writing and in careful speech

You can absolutely say Kada je dan sunčan, ne nosim kišobran.
The meaning does not change; it just sounds slightly more formal or complete.

Why is the word order Kad je dan sunčan and not Kad dan je sunčan or Kad je sunčan dan?

The basic neutral order for "the day is sunny" is:

Dan je sunčan. (Subject – verb – complement)

In a "when"-clause, we just put kad / kada in front:

Kad je dan sunčan, …

The other versions:

Kad dan je sunčan – sounds wrong/ungrammatical. The verb je normally stays right after kad or after the subject, not between subject and its adjective like this.
Kad je sunčan dan – is grammatically possible, but it sounds a bit more stylistic/poetic, like "when (there) is a sunny day" rather than the more neutral "when the day is sunny". The original is the most standard everyday version.

Why is the verb je (is) necessary? Could I say Kad dan sunčan, ne nosim kišobran?

You must use je here. Croatian, unlike some Slavic languages in the past tense, does not normally drop the present form of biti (to be) in sentences like this.

Correct: Kad je dan sunčan, …
Incorrect: Kad dan sunčan, …

Leaving out je would sound like broken or foreigner’s Croatian in the present tense.

What tense is nosim and why is it used instead of something like neću nositi?

Nosim is present tense, 1st person singular of nositi (to carry).

In Croatian, the present tense is used for:

habits, regular actions: Ne nosim kišobran. = I don’t carry / I don’t (usually) take an umbrella.
general truths or repeated situations: Kad je dan sunčan, ne nosim kišobran.

If you said:

Kad je dan sunčan, neću nositi kišobran.

that would mean When the day is sunny, I *will not carry an umbrella* — more specifically about the future or a decision, not a general habit.

Why is there no ja (I) in ne nosim kišobran? How do we know it means I?

In Croatian, the verb ending usually shows the person, so the subject pronoun is often dropped.

nosim ends in -m, which marks 1st person singular (I).
So (ja) ne nosim kišobran clearly means I don’t carry an umbrella.

You can say Ja ne nosim kišobran if you want to emphasize I (contrast like I don’t, but they do), but in a neutral sentence it is more natural to omit ja.

Why is kišobran not changed in form here? Is it in accusative case?

Yes, kišobran here is in the accusative case, because it is the direct object of nosim (I carry what? → umbrella).

For many masculine nouns ending in a consonant, the nominative singular and accusative singular look the same:

Nominative: kišobran (as a subject: Kišobran je crn. – The umbrella is black.)
Accusative: kišobran (as an object: Nosim kišobran. – I carry an umbrella.)

The form is identical, but its function in the sentence (subject vs object) is different.

Why is there a comma after sunčan? Is a comma always needed before kad-clauses?

Here, Kad je dan sunčan is a subordinate clause (the “when”-part) and ne nosim kišobran is the main clause.

Croatian punctuation rules:

When the subordinate clause comes first, you normally separate it from the main clause with a comma:
Kad je dan sunčan, ne nosim kišobran.
If you reverse the order (main clause first), you usually still keep the comma:
Ne nosim kišobran kad je dan sunčan.

So yes, in sentences like this, you normally put a comma between the kad-clause and the main clause.

What is the difference between Kad je dan sunčan and Kad je sunčano?

Both are correct but slightly different in nuance:

Kad je dan sunčan – literally When the day is sunny.
Focuses on the day (a masculine noun dan) and describes it with the adjective sunčan.
Kad je sunčano – literally When it is sunny.
sunčano is a neuter adverbial/adjectival form used impersonally, like English it’s sunny (with dummy it).

In everyday speech, Kad je sunčano, ne nosim kišobran might be even more common and sounds a bit lighter and more colloquial. The original sentence is slightly more formal or structured, but both are natural.

Why is the adjective placed after the noun: dan sunčan, and not before, like sunčan dan?

Both orders are possible, but they have different typical uses:

sunčan dan (adjective before noun) is the most common, neutral way to say a sunny day (e.g. Volim sunčan dan.).
dan sunčan (adjective after noun) is less common and feels a bit more formal, descriptive, or stylistic. It’s often used:
- in more literary or careful language,
- when emphasizing the state: the day is sunny.

In Kad je dan sunčan, the structure is like saying when the day is sunny (as a state), so the post-nominal adjective fits. You could also say:

Kad je dan sunčan, ne nosim kišobran. (as given)
Kad je sunčan dan, ne nosim kišobran. (also possible, but slightly different rhythm/emphasis)

Both are understandable; the given version sounds like a straightforward descriptive clause.

Does kad here mean whenever (general habit) or when for a specific time?

In this sentence, Kad je dan sunčan, ne nosim kišobran is understood as a general rule / habit, so English would most naturally translate it as:

When it’s sunny, I don’t carry an umbrella.
or
Whenever it’s sunny, I don’t carry an umbrella.

Croatian kad / kada + present tense is often used for whenever-type general conditions, especially when combined with a present tense in the main clause. Context will usually tell you if it’s general (whenever) or specific (when, on that particular occasion). Here, the habitual present ne nosim clearly suggests a general habit.