Learn Hardcore Croatian: Ponekad me boli glava ujutro. - Sometimes my head hurts in the morning.

me is the short, unstressed accusative pronoun (object form of “I”). The verb boljeti “to hurt/ache” takes the person who feels the pain in the accusative.
mi is dative and is used with other impersonal feelings like Hladno mi je (I feel cold). It’s not used with boljeti.
mene is the full (stressed) accusative form, used for emphasis: Mene ponekad boli glava ujutro (emphasis on “me”).
Natural: Ponekad me boli glava ujutro.
Wrong with this verb: Ponekad mi boli glava ujutro.

What is the subject here, and why is boli singular?

The grammatical subject is glava (head), which is nominative singular.
The verb boli is 3rd person singular present and agrees with glava.
The experiencer (me) is the direct object (accusative).
Compare plural body parts: Oči me bole (My eyes hurt), Leđa me bole (My back hurts — leđa is plural).

Why isn’t it boli glavu or boli glavom?

glava is the subject, so it stays in nominative: glava.
glavu (accusative) would make “head” the object, which isn’t the structure in Croatian for pain.
glavom (instrumental) would mean “with the head.”

Why is me right after Ponekad? Could I say Ponekad boli me glava?

Short pronouns like me are clitics and normally sit in “second position” in the clause (after the first stressed word).
After Ponekad the clitic me comes: Ponekad me boli glava ujutro.
Ponekad boli me glava is unnatural in standard Croatian because the clitic isn’t in second position.
You also can’t start a clause with a clitic: avoid Me boli glava (wrong).

Can I front glava or change the word order?

Yes, word order is flexible for emphasis as long as clitics keep their position:

Neutral: Boli me glava (ujutro).
Emphasis on “head”: Glava me boli (ujutro).
Time early: Ujutro me boli glava.
Both adverbs: Ponekad me ujutro boli glava or Ujutro me ponekad boli glava.

Do I need to say my head (moja glava)?

Usually no. Croatian uses the body-part-as-subject plus object-pronoun:

Natural: Boli me glava.
With a possessive for contrast/emphasis only: Boli me moja glava, a tebe bole leđa. In everyday talk, it sounds heavy or contrastive.

What’s the difference between this and Imam glavobolju?

Boli me glava is the most common conversational way to say “I have a headache.”
Imam glavobolju (I have a headache) is more formal/medical or careful style.
Both are correct; choose based on tone and context.

How do I negate it?

Put ne before the verb; clitics keep their position:
- Ujutro me ne boli glava. (In the morning my head doesn’t hurt.)
- Ne boli me glava ujutro.
- Ponekad me ujutro ne boli glava. (Sometimes it doesn’t hurt in the morning.)

How do I ask a yes/no question?

Use li after the verb; move the clitic accordingly:
- Boli li te glava ujutro? (Does your head hurt in the morning?)
Don’t use je li here, because boljeti doesn’t take an auxiliary in the present tense.

What about other body parts, especially plural ones?

The verb agrees with the body part:
- Singular: Ruka me boli. (My arm hurts.)
- Plural: Ruke me bole. (My arms hurt.)
- Pluralia tantum: Leđa me bole. (My back hurts.)

What’s the difference between ujutro, jutros, and u jutro?

ujutro = in the morning (habitual/general time): Ujutro me boli glava.
jutros = this morning (today): Jutros me je boljela glava.
u jutro (two words) is generally avoided; standard usage is the fused adverb ujutro.

Is ponekad the same as katkad or nekad?

ponekad and katkad are near-synonyms for “sometimes.” Ponekad is more common.
nekad(a) can mean “sometimes” in colloquial speech, but also “once/used to/in the past,” so it can be ambiguous. Prefer ponekad for clarity.

Can I express the onset of pain (starts to hurt)?

Yes, use the perfective zaboljeti:

Ponekad me ujutro zaboli glava. (Sometimes my head starts hurting in the morning.)
Ongoing state uses boljeti: Ponekad me ujutro boli glava.

Can I drop me and just say Ponekad boli glava ujutro?

Grammatically possible, but it no longer means “my head hurts.” It becomes a generic statement about “the head” hurting (odd without context) or about someone else’s head. To say “I have a headache,” keep me.