Learn Hardcore Croatian: Navečer me bole leđa ako dugo sjedim u uredu. - In the evening my back hurts if I sit in the office for a long time.

Because the verb boljeti agrees with what hurts, which is the subject. Leđa is grammatically plural, so the verb must be bole (3rd person plural). Compare:

Boli me glava. (singular subject: glava)
Bole me oči. (plural subject: oči)
Bole me leđa. (plural-only noun: leđa)

What case is me? Why not mi?

me is the unstressed accusative of ja used with boljeti to mark the experiencer. The dative mi is not used here in standard Croatian. For emphasis you can use the stressed form mene: Mene bole leđa.

Why is leđa treated as plural?

In Croatian, leđa (“back”) exists only in the plural (it’s a pluralia tantum noun). Key forms:

Nominative/Accusative/Genitive plural: leđa
Dative/Locative/Instrumental plural: leđima Examples: Bole me leđa. / Bol u donjem dijelu leđa. / U leđima me boli.

Can I say Boli me u leđima instead?

Yes. Two natural options:

Bole me leđa. Subject is leđa (plural); focuses on the body part itself.
Boli me u leđima. Uses singular boli with a prepositional phrase; focuses on the location of pain. For lower-back pain in Croatia you’ll also hear Bole me križa.

Is the word order fixed? Where does the clitic me have to go?

Clitic pronouns like me generally sit in the “second position” of the clause (after the first stressed word/phrase). Natural variants:

Navečer me bole leđa.
Leđa me bole navečer.
Ako dugo sjedim u uredu, navečer me bole leđa. Avoid placing me first or too late in the clause (e.g., ?Navečer bole me leđa sounds off).

Should there be a comma before ako?

Yes in standard punctuation: subordinate ako-clauses are set off by a comma. So: Navečer me bole leđa, ako dugo sjedim u uredu. If the ako-clause comes first, put the comma after it: Ako dugo sjedim u uredu, navečer me bole leđa. (In informal writing you may see the comma omitted, but include it in careful/formal text.)

Why use ako and not kad?

ako = “if” (a condition that may or may not happen).
kad = “when/whenever” (time/habit). For habitual cause–effect, kad often sounds more natural: Navečer me bole leđa, kad dugo sjedim u uredu. With ako, you phrase it as a condition.

Why is it u uredu and not u ured?

Because with static location u takes the locative case: u uredu (“in the office”). With motion toward a place it takes the accusative: u ured (“into the office”).

Location: Sjedim u uredu.
Direction: Idem u ured. Basic singular forms of ured: ured (N), ureda (G), uredu (D/L), ured (A), uredom (I).

Could I say na poslu instead of u uredu?

Yes, but the nuance changes:

u uredu = “in the office” (the physical office/room).
na poslu = “at work” (at your workplace/job in general, not necessarily an office). Use whichever fits your situation.

What’s the difference between navečer, uvečer, and večeras?

navečer and uvečer both mean “in the evening.” In Croatia, navečer is more common/preferred; uvečer is also understood.
večeras means “this evening/tonight” (specific to today). For habitual meaning you can also say svake večeri (“every evening”).

Why is it sjedim and not sjesti? Aren’t they the same verb?

They’re related but not the same:

sjediti = to be sitting (a state; imperfective). Present: sjedim, sjediš, sjedi…
sjesti = to sit down (the act of taking a seat; perfective). You don’t use sjesti to describe the ongoing state of sitting. There’s also sjedati (iterative “to be sitting down repeatedly”).

Is dugo enough, or should I say dugo vremena?

dugo by itself is perfectly natural for “for a long time.” dugo vremena adds emphasis. For “too long,” use predugo. Word order is flexible: ako dugo sjedim / ako sjedim dugo (both fine).

Is kancelarija acceptable instead of ured?

In Croatian, ured is the neutral, standard word for “office.” kancelarija exists but is less common in Croatia and can sound regional/bureaucratic or more typical in Serbian/Bosnian usage. For standard Croatian, prefer ured.