Learn Hardcore Croatian: Kad imam alergiju, često kihnem i dobijem lagan kašalj. - When I have an allergy, I often sneeze and get a light cough.

The dictionary form alergija is nominative singular (used for the subject).
After imati (to have), the thing you “have” is in the accusative case, because it is a direct object.

Alergija is a feminine noun in -a, and feminine nouns like this have accusative singular in -u:

Nominative: alergija (Allergy is a problem.)
Accusative: alergiju (I have an allergy.)

So imam alergiju literally means “I-have allergy (as direct object).”

What tense and aspect are imam, kihnem, and dobijem, and why are they all in the present tense?

All three verbs are in the present tense, but they differ in aspect:

imam – present tense, imperfective aspect (imati = to have)
kihnem – present tense, perfective aspect (kihnuti = to sneeze once)
dobijem – present tense, perfective aspect (dobiti = to get, receive)

In Croatian, you can use present tense to talk about:

What is happening now
General truths or habitual actions (“whenever this happens, that happens”)

The clause with kad introduces a whenever/when condition:

Kad imam alergiju = When(ever) I have an allergy

Then for each such situation, kihnem and dobijem describe the individual events that happen in that situation. Perfective present here expresses something like “I (typically) sneeze (at that point)” and “I (then) get a mild cough.”

So the whole sentence is a habitual/generic statement, even though the verbs are just in the normal present tense.

What is the infinitive of kihnem, and how is it conjugated?

The infinitive is kihnuti (to sneeze once). It is a perfective verb. Present tense:

ja kihnem – I sneeze (once)
ti kihneš – you sneeze
on/ona/ono kihne – he/she/it sneezes
mi kihnemo – we sneeze
vi kihnete – you (pl./formal) sneeze
oni/one/ona kihnu – they sneeze

The corresponding imperfective verb (for ongoing or repeated sneezing) is kihati:

ja kiham – I am sneezing / I (tend to) sneeze
ti kihaš
on/ona/ono kiha
mi kihamo
vi kihate
oni/one/ona kihaju

In your sentence, kihnem (perfective) fits well because it refers to each individual sneeze that happens when you have an allergy.

Why is kihnem used instead of kiham in this sentence? Could I say Kad imam alergiju, često kiham…?

Both are possible, but they have slightly different nuances:

Često kihnem – suggests distinct, countable sneezes that occur when you have an allergy (each time you have an allergy episode, you sneeze).
Često kiham – focuses more on the ongoing action of sneezing (you’re in a state of sneezing a lot when you have an allergy).

Your sentence:

Kad imam alergiju, često kihnem i dobijem lagan kašalj.
= Whenever I have an allergy, I (end up) sneezing and getting a mild cough.

You could also say:

Kad imam alergiju, često kiham i kašljem.
= When I have an allergy, I often sneeze and cough (in general, ongoingly).

So kihnem is perfectly fine and sounds very natural; kiham is also correct, but shifts the focus a bit toward the continuous activity.

How does kad work here? Is it the same as kada, and why isn’t a future tense used?

kad is a shortened, more colloquial form of kada. In meaning, they are the same: both mean “when.”

In your sentence:

Kad imam alergiju = When(ever) I have an allergy

With kad/kada used for general time or repeated situations, Croatian normally uses the present tense in both clauses, even though in English we might think of it as “whenever”:

Kad imam alergiju, često kihnem…
= Whenever I have an allergy, I often sneeze…

For future situations, Croatian also typically keeps the subordinate clause in the present, and uses a future only in the main clause:

Kad budem imao alergiju, uzet ću lijek.
= When I (will) have an allergy, I’ll take medicine.

So not: Kad ću imati alergiju (this is usually wrong or very marked).

Why is lagan in lagan kašalj in that form? What case is it, and how does it agree with kašalj?

Kašalj (cough) is a masculine noun. In the sentence:

dobijem lagan kašalj

kašalj is the direct object of dobijem, so it is in the accusative case. However, for inanimate masculine nouns, the accusative singular is identical to the nominative singular:

Nominative: kašalj
Accusative: kašalj

The adjective lagan must agree in gender, number, and case with the noun:

masculine
singular
accusative (same form as nominative for this type)

So the correct form is:

lagan kašalj

You might also see lagani kašalj in some styles; both are used, with very similar meaning, but lagan kašalj is short and natural here.

Could I say dobijem laganu kašalj or dobijem laganog kašlja instead?

No, not in this exact structure:

dobijem laganu kašalj – incorrect
- laganu is feminine accusative singular, but kašalj is masculine, so the adjective and noun don’t agree in gender.
dobijem laganog kašlja – grammatically possible, but it sounds odd here and changes the structure
- laganog kašlja is genitive singular (of a mild cough).
- This kind of genitive is sometimes used in “partitive” expressions (e.g. malo kašlja = a bit of cough), but with dobiti it’s much more natural to keep the direct object in the accusative:
  - dobijem lagan kašalj

So in your sentence, dobijem lagan kašalj is the normal and idiomatic choice.

What is the difference in meaning between lagan kašalj, blagi kašalj, and mali kašalj?

All three can be used to describe a non-severe cough, but with slightly different nuances:

lagan kašalj – literally light cough
- Emphasizes that it is not heavy, not intense. Very natural in everyday speech.
blagi kašalj – literally mild cough
- A bit more neutral or medical-sounding. Common in doctors’ descriptions or more formal speech.
mali kašalj – literally small/little cough
- Less common; can sound a bit childish or imprecise. You’re more likely to hear malo kašlja (“a bit of cough”) than mali kašalj.

In your sentence, lagan kašalj and blagi kašalj are both very natural; lagan kašalj is slightly more colloquial.

Where can često go in the sentence? Are other word orders also correct?

Yes, često (often) is fairly flexible. All of these are grammatical, with slightly different emphasis:

Kad imam alergiju, često kihnem i dobijem lagan kašalj.
– Neutral: often, I sneeze and get a mild cough.
Kad imam alergiju, kihnem često i dobijem lagan kašalj.
– Emphasizes the frequency of the sneezing more strongly.
Kad imam alergiju, kihnem i često dobijem lagan kašalj.
– Suggests that the cough is what happens “often” (maybe more focus on the cough than the sneeze).
Često kad imam alergiju, kihnem i dobijem lagan kašalj.
– Emphasizes that often, when you have an allergy episode, this whole pattern happens.

The version you have (Kad imam alergiju, često kihnem...) is the most straightforward and natural.

Why is there a comma after Kad imam alergiju? Is a comma always required with kad?

In Croatian, when a sentence starts with a subordinate clause introduced by kad/kada, you normally put a comma before the main clause:

Kad imam alergiju, često kihnem i dobijem lagan kašalj.

This is standard punctuation:

[Subordinate clause], [Main clause].

If you reverse the order, the comma is usually not used:

Često kihnem i dobijem lagan kašalj kad imam alergiju.

So:

Kad … , [main clause] → comma
[Main clause] kad … → usually no comma.

How do you pronounce kašalj, and what happens with the final lj?

Pronunciation of kašalj:

ka – like “kah” in “karma”
š – like English “sh” in sheep
alj – one syllable, ending with the palatal lj sound

The letter š is always “sh”.
The letter pair lj is a single consonant in Croatian, pronounced like the “lli” in British “million” or similar to the Spanish ll in some accents.

Phonetically, kašalj is roughly:

[kȁ-ʃaʎ] (stress on the first syllable)

So you do not pronounce l and j separately; they fuse into one [ʎ] sound.

Could this sentence be expressed with a clear future meaning, like Kad ću imati alergiju, često ću kihnuti…?

That version is not natural in Croatian. With kad/kada introducing a future condition, Croatian prefers:

present or future perfect in the kad-clause
future tense in the main clause

For example:

Kad budem imao alergiju, često ću kihnuti i dobit ću lagan kašalj.
= When I (will) have an allergy (in the future), I will often sneeze and I’ll get a mild cough.

Or more simply, if it’s a general habit rather than a specific future situation, you just keep the entire sentence in the present, as in your original:

Kad imam alergiju, često kihnem i dobijem lagan kašalj.

So Kad ću imati alergiju is normally avoided; Croatian does not mirror the English “When will I have…?” structure in this kind of conditional sentence.