Learn Hardcore Croatian: Kad me boli glava, ostajem kod kuće. - When I have a headache, I stay at home.

Both mean “when.” Kad is the shorter, very common form; kada is the full form, a bit more formal or used for rhythm/emphasis. In this sentence, both are correct: Kad me boli glava… = Kada me boli glava…

Could I use dok or ako instead of kad?

dok = “while,” focusing on simultaneity (While my head hurts, I stay at home) — odd here because a headache is the reason, not just simultaneous background time.
ako = “if,” a condition (If I have a headache, I stay at home) — this changes the meaning to conditional. Stick with kad for “when/whenever.”

Why is there a comma after the first clause? Do I also need a comma if I reverse the order?

When the dependent clause comes first, Croatian normally uses a comma: Kad me boli glava, ostajem kod kuće.
If you reverse the order, you usually do not use a comma: Ostajem kod kuće kad me boli glava. A comma there would sound parenthetical or emphatic.

Why me and not mi or mene? What case is it?

me = unstressed accusative (“me”), required by the verb boljeti (to hurt) because the body part is the subject and the person is the object.
mi = dative (“to me”), not standard here.
mene = stressed/long accusative for emphasis: Mene boli glava (“It’s me who has the headache”).
So standard is Boli me glava.

Where does me go in the sentence? Could I move it?

Unstressed pronouns like me are Wackernagel clitics: they like the second position in their clause. Examples:

Kad me jako boli glava, … (correct: me comes right after Kad)
Kad me glava boli, … (also fine)
Boli me glava. / Glava me boli. (both fine) Avoid: Kad boli me glava (sounds wrong).

Is glava the subject? Why is it nominative and not something like glavu?

Yes, glava is the subject in nominative; me is the (accusative) object. The pattern is:
“[Body part in nominative] + boli + [person in accusative].”
Plural changes the verb: Bole me zubi (“My teeth hurt”).

Could I say moja glava?

Normally no. Croatian usually omits possessives with body parts when the possessor is obvious from the object pronoun (me). Boli me glava is natural; Boli me moja glava sounds contrastive or odd unless you really mean “my head (as opposed to someone else’s).”

Is Kad me boli glava the same as Kad imam glavobolju?

They’re close in meaning. Boli me glava is the most natural everyday way to say “I have a headache.” Imam glavobolju is perfectly correct but feels more formal/medical or matter-of-fact.

What’s the difference between boli and zaboli?

boli (imperfective) = is/was hurting (ongoing state).
zaboli (perfective) = starts to hurt (onset).
So: Kad me zaboli glava, highlights the moment it begins; Kad me boli glava, describes the state in general.

Why ostajem and not ostanem or ću ostati?

ostajem is the present of the imperfective verb ostajati and is used for habits: “I (usually) stay.”
ostanem is the present of the perfective ostati and typically refers to a single future-like event or conditional context; it doesn’t express a general habit well.
ću ostati = “I will stay” (future, one-time or planned).

What does kod kuće literally mean, and why not u kući?

kod + genitive means “at someone’s place,” so kod kuće is an idiom for “at home.”
u kući = “in the house” (inside the building), which is more spatial and not the default way to say “at home.”

Is kući without kod correct? What about doma?

kući (without kod) is widely used and acceptable in Croatian as an adverb meaning “at home”: Ostajem kući.
doma is also standard and very common: Ostajem doma.
All three are fine; kod kuće is a bit more neutral/formal, kući/doma more colloquial/everyday (regional preferences apply).

Can I say Kad god me boli glava?

Yes. kad god = “whenever” and emphasizes habituality: Kad god me boli glava, ostajem kod kuće.

Can I include ja (I), or is it dropped?

You usually drop it: Ostajem kod kuće.
Add ja for emphasis or contrast: Ja ostajem kod kuće (as opposed to someone else).

Are there common word-order variants of the headache part?

Yes, all natural, with slight emphasis differences:

Boli me glava. (neutral)
Glava me boli. (emphasis on the body part)
In a clause: Kad me boli glava… / Kad me glava boli…
Avoid Boli mi glava in standard Croatian (it’s a regional/colloquial variant).