На ладони остался маленький синяк, но царапина уже почти зажила.

На ладони literally means on the palm or on the surface of the palm.

Russian uses на when something is on a surface, and в when something is inside something.

So here:

• на ладони = on the palm of the hand
• в ладони would suggest in the palm / inside a cupped hand, which is a different idea

Because a bruise or scratch is located on the skin of the palm, на is the natural choice.

It is in the prepositional case.

The base noun is:

• ладонь = palm of the hand

After на when talking about location, Russian usually uses the prepositional case:

• на ладони = on the palm

So:

• nominative: ладонь
• prepositional: ладони

This is why you see на ладони, not на ладонь.

Остался is the past tense of остаться, which means to remain, to stay, or to be left.

In this sentence:

• На ладони остался маленький синяк = A small bruise remained on the palm / There was still a small bruise on the palm

Russian often uses остаться where English might simply say there was still or a bruise was left.

The form остался is:

• past tense
• masculine singular

It agrees with синяк, which is a masculine noun.

Because синяк is the subject of the clause.

In:

• На ладони остался маленький синяк

the thing that remained is маленький синяк.

So:

• синяк = nominative singular
• маленький matches it in gender, number, and case

Even though the sentence starts with На ладони, that phrase is just telling you where. The actual grammatical subject is маленький синяк.

Russian word order is much more flexible than English word order.

The sentence:

• На ладони остался маленький синяк

sounds natural because Russian often places:

1. the location/background information first
2. then the verb
3. then the thing being talked about

This gives a natural flow:
On the palm remained a small bruise.

In more neutral textbook-style order, you could also say:

• Маленький синяк остался на ладони.

But the original version is very natural and highlights the location first.

Usually, yes: синяк means bruise.

But in informal Russian it can sometimes also mean:

• a black eye
• a dark bruise-like mark

In this sentence, it clearly just means bruise.

A useful note:

• синий means blue
• синяк is related to that idea, since bruises often look bluish

Зажила is the past tense of зажить, which means to heal up.

So:

• царапина = scratch
• царапина уже почти зажила = the scratch has already almost healed

The verb is feminine singular past because царапина is a feminine noun.

Forms of the past tense depend on gender:

• masculine: зажил
• feminine: зажила
• neuter: зажило
• plural: зажили

Since царапина is feminine, Russian uses зажила.

Russian aspect matters a lot here.

• заживать = imperfective = to be healing / to heal over time
• зажить = perfective = to heal up / become healed

In the sentence:

• царапина уже почти зажила

the speaker is talking about the scratch as being close to a completed result. That is why the perfective verb зажить is used.

If you said:

• царапина заживала

that would mean something more like:

• the scratch was healing
• focusing on the process, not the near-complete result

So зажила fits better with almost healed.

Уже почти means already almost.

• уже = already
• почти = almost

Together they show that the healing is well advanced:

• царапина уже почти зажила = the scratch has already almost healed

This can sound a little unusual if translated word-for-word into English, but it is completely natural in Russian.

It suggests something like:

• by now, it is nearly healed
• it has progressed to the point where healing is almost complete

Russian has no articles.

So nouns like:

• синяк
• царапина

can mean:

• a bruise / the bruise
• a scratch / the scratch

You understand which one is meant from context.

In this sentence, English naturally uses:

• a small bruise
• the scratch

But Russian does not need separate words for that.

Но means but.

It connects two contrasting ideas:

• На ладони остался маленький синяк = a small bruise remained
• но царапина уже почти зажила = but the scratch had almost healed

So the contrast is:

• one mark is still there
• the other is nearly gone

This is a very common use of но.

Yes, that is a good literal translation.

More natural English would usually be:

• There was still a small bruise on the palm, but the scratch had almost healed.
• A small bruise remained on the palm, but the scratch had almost healed.

Literal translation helps show the structure:

• На ладони = on the palm
• остался = remained
• маленький синяк = a small bruise
• но = but
• царапина = the scratch / a scratch
• уже почти зажила = had already almost healed / had nearly healed

So the literal version is fine for learning grammar, even if it sounds less natural in English.