Lijeva ruka me još boli.

Because ruka is a feminine singular noun, and the adjective has to agree with it in gender, number, and case.

• lijeva ruka = left hand/arm
• masculine would be lijevi
• neuter would be lijevo

So:

• lijeva ruka = left hand/arm
• lijevi lakat = left elbow
• lijevo koljeno = left knee

The adjective and noun must match.

Because ruka is the subject of the sentence, so it stays in the nominative case.

This sentence is built like:

• Lijeva ruka = the thing that hurts
• me = the person affected
• boli = hurts

So literally, the structure is close to:

• Left hand hurts me.

That is why ruka is nominative, not accusative.

You would see ruku in other roles, for example:

• Vidim ruku. = I see a hand.
• Podižem ruku. = I am raising my hand.

There, ruku is an object, so it is accusative.

Because the verb boljeti works differently from English to hurt.

In Croatian, the thing that hurts is usually the subject, and the person feeling the pain is expressed with an object pronoun:

• Boli me glava. = My head hurts.
  Literally: The head hurts me.
• Boli me ruka. = My arm/hand hurts.

So in Lijeva ruka me još boli, me means me in the sense of it hurts me.

Because me is the normal short unstressed form of the pronoun, and that is what Croatian usually uses in sentences like this.

• me = short clitic form
• mene = full/strong form

In ordinary neutral speech, you usually say:

• Boli me ruka.

You would use mene only for emphasis, contrast, or in certain positions:

• Mene još boli lijeva ruka, ali njega više ne boli.
  = My left hand still hurts, but his doesn’t anymore.

So in your sentence, me is the natural choice.

Because me is a clitic, and Croatian clitics usually go in the second position in the sentence or clause.

Here, the first unit is Lijeva ruka, so the clitic me comes right after it:

• Lijeva ruka me još boli.

This is very typical Croatian word order behavior.

Compare:

• Ruka me boli.
• Lijeva ruka me boli.
• Ta me ruka još boli.

In each case, the clitic tends to appear after the first phrase.

Boli is the 3rd person singular present tense of boljeti = to hurt / to ache.

It is 3rd person singular because the subject is singular:

• ruka = singular
• therefore boli = singular

Compare:

• Ruka me boli. = My hand/arm hurts.
• Noge me bole. = My legs hurt.

So:

• singular subject → boli
• plural subject → bole

Here još means still.

So:

• Lijeva ruka me još boli. = My left hand still hurts.

Depending on context, još can also mean more, another, or yet, but in this sentence the natural meaning is still.

Examples:

• Još čekam. = I’m still waiting.
• Treba mi još vremena. = I need more time.
• Još jedan! = One more!

Yes, Croatian word order is fairly flexible, but not all versions sound equally neutral.

Your sentence:

• Lijeva ruka me još boli.

is perfectly natural.

You could also hear:

• Još me boli lijeva ruka.
• Lijeva me ruka još boli.
• Boli me još lijeva ruka.

These can sound slightly different in emphasis or style.

However, because of clitic placement, you cannot move me completely freely. Croatian word order is flexible, but clitics like me have special placement rules.

Because Croatian often omits possessives like my, your, his, etc. when the meaning is obvious from context.

So instead of saying:

• Moja lijeva ruka me još boli.

Croatian often simply says:

• Lijeva ruka me još boli.

The possessive moja is possible, but it is usually added only if you want to emphasize it, contrast it, or make it especially clear.

For example:

• Moja lijeva ruka me još boli, a desna je dobro.
  = My left hand still hurts, but the right one is fine.

It can mean hand or arm, depending on context.

This is something that often surprises English speakers, because English separates the two more strictly.

In Croatian:

• ruka can refer to the whole arm/hand area
• if you need to be more specific, context usually tells you
• sometimes a more precise phrase is used if necessary

So Lijeva ruka me još boli could mean:

• My left hand still hurts
• or My left arm still hurts

depending on the situation.

Yes, absolutely. That is a very common and natural version.

Compare:

• Boli me lijeva ruka.
• Lijeva ruka me boli.
• Lijeva ruka me još boli.

All are grammatical. The difference is mostly about focus and word order.

• Boli me lijeva ruka. often sounds like the most straightforward way to say it.
• Lijeva ruka me boli. puts more focus on the left hand/arm.
• Lijeva ruka me još boli. adds the idea of still.

Yes. Croatian has no articles.

So where English says:

• the left hand
• a hand

Croatian simply says:

• lijeva ruka
• ruka

Context tells you whether it means the, a, or sometimes just a general idea.

That is why Lijeva ruka me još boli is fully complete without any word corresponding to the.