Learn Hardcore Croatian: Danas me bole leđa.

Because leđa (back) is a plural-only noun (pluralia tantum) in Croatian. It’s treated as neuter plural, so the verb agrees in 3rd person plural: bole.
Compare: Boli me zub (tooth, singular), but Bole me zubi (teeth, plural) and Bole me leđa (back, plural-only).

What case is me, and why is it used here?

me is the unstressed accusative form of ja (I). With boljeti, the body part is the grammatical subject, and the experiencer (the person who feels pain) is in the accusative: Boli me glava, Bole me leđa.

Why not mi instead of me?

mi is the dative clitic (“to me”). Standard Croatian uses the accusative (me/te/ga…) with boljeti, so say Boli me glava, not Boli mi glava. You will, however, use dative with states like weather/temperature: Hladno mi je, Meni je vruće. Some speakers use dative with boljeti colloquially, but avoid it in standard Croatian.

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

me is a clitic and wants to be in “second position” in its clause (after the first stressed word/phrase). Natural options:

Danas me bole leđa.
Leđa me bole danas.
Bole me leđa danas.
Danas me jako bole leđa. Avoid placing the clitic later than second position: ✗ Danas leđa me bole sounds wrong.

Can I say the same thing without starting with Danas?

Yes. The core is Bole me leđa. Adding danas just sets the time. You can also put it at the end for emphasis: Bole me leđa danas.

Why don’t we say “my” (moja) here?

With body parts, Croatian typically uses a pronoun for the person and leaves the body part bare: Boli me glava, Bole me leđa. If you need emphasis on ownership, you can say Moja leđa me bole, but it’s emphatic/marked.

What does leđa exactly refer to, and how does it decline?

leđa = back (the whole back area). It’s plural-only. Key forms:

Nominative/Accusative plural: leđa (subject/object)
Dative/Locative/Instrumental plural: leđima (e.g., na leđima, u leđima) Because it’s plural-only, it triggers plural agreement: Bole me leđa.

How do you pronounce and type the letter đ in leđa?

Pronunciation: a soft palatalized D, close to the “dy” in “would you” said quickly. It’s not the same as dž (which sounds like English “j” in “jam”).
Typing: If you lack the Croatian layout, you can write dj as a fallback (ledja), but the correct spelling is leđa.

Is it wrong to say Danas me boli leđa?

In standard Croatian, yes—use plural bole because leđa is plural. You may hear singular boli in casual speech from some speakers, but avoid it in correct usage.

How do I say it in the past: “Yesterday my back hurt”?

Jučer su me boljela leđa.
Explanation: The auxiliary su (they are) + past participle boljela agrees with plural leđa.

How do I negate it?

Danas me ne bole leđa.
The negative particle ne goes before the verb; the clitic me still stays in second position.

How do I ask someone “Do your back hurt (today)?” formally and informally?

Informal: Bole li te leđa (danas)?
Formal/plural you: Bole li vas leđa (danas)?
In Croatian, Bole li… is preferred over Da li… in the standard.

Is boljeti reflexive? Should I add se?

No. boljeti is not reflexive here, so don’t use se. Say Bole me leđa, not ✗ Bole me se leđa.

How do I talk about other body parts with the same pattern?

Boli me glava. (head, singular → boli)
Boli me zub. (tooth, singular → boli)
Bole me oči. (eyes, plural → bole)
Bole me koljena. (knees, plural → bole)

How can I intensify or specify the pain?

Intensity: Danas me jako/strašno/užasno bole leđa.
Location: Boli me donji dio leđa. (lower back; subject is singular dio, so boli)
Alternative phrasing: Boli me u leđima; also acceptable: Imam bol u leđima / Imam bolove u leđima (more clinical/formal).

What prepositions commonly go with leđa?

na leđima (on my back): Nosim ruksak na leđima.
u leđima (in my back area): Boli me u leđima.
s leđa / iza leđa (from behind / behind one’s back): Napali su ga s leđa. / Govori iza mojih leđa.

What if I mean the spine specifically, not the back?

Use kralježnica (spine) in Croatian: Boli me kralježnica.
You’ll also hear kičma in the region (standard in Serbian/Bosnian); in Croatian it’s colloquial/regional.