Хотя соло было коротким, зрители запомнили именно его.

Хотя means although / even though. It introduces a concessive clause: a clause that presents something surprising in contrast to the main result.

So in this sentence:

Хотя соло было коротким = Although the solo was short
зрители запомнили именно его = the audience remembered that specifically

The idea is: you might expect a short solo to be less memorable, but it was actually the thing the audience remembered.

Because Хотя соло было коротким is a subordinate clause, and in Russian subordinate clauses are separated from the main clause by a comma.

So the structure is:

Хотя... , ...

This is very normal in Russian:

• Хотя было поздно, он не ушёл.
• Хотя фильм длинный, он не скучный.

The comma is required.

Because соло is treated as a neuter singular noun in Russian.

It is also an indeclinable loanword, meaning its form usually does not change by case:

• это соло
• без соло
• о соло

Since соло is neuter singular, the past-tense form of быть agrees with it as было:

• он был
• она была
• оно было
• они были

So:

соло было коротким = the solo was short

This is a very common learner question.

Here коротким is the instrumental singular form of короткий. After быть in the past tense, Russian often uses a predicate adjective in the instrumental:

• Фильм был интересным.
• Ночь была холодной.
• Соло было коротким.

You may also hear короткое in some contexts, especially in more conversational speech, but коротким is completely standard and natural.

So the sentence uses a very normal pattern:

[subject] + был/была/было + adjective in instrumental

Запомнить means something like to remember, to retain in memory, to have something stick in your mind. It often focuses on the result: the audience came away remembering it.

So:

• помнить = to remember, to have in memory
• запомнить = to remember successfully, to retain, to memorize, to notice and remember

Here запомнили works well because the idea is not just that the audience possessed a memory in general, but that this solo was the part that stuck with them.

Also, запомнили is perfective past plural:

• perfective: completed result
• past: happened in the past
• plural: agrees with зрители

Because его is the form used for a direct object, while оно is the subject form.

Compare:

• Оно было коротким. = It was short.
  Here оно is the subject.

• Зрители запомнили его. = The audience remembered it.
  Here его is the direct object.

Since соло is the thing being remembered, Russian uses его.

A useful shortcut:

• оно = it as subject
• его = it as object

Russian often avoids repeating a noun when it is already clear from context.

So after соло has already been mentioned, it is very natural to say:

зрители запомнили именно его

instead of repeating соло again.

Using the pronoun makes the sentence smoother and also helps the emphasis land strongly on его, especially with именно.

You could repeat the noun, but it would sound a bit heavier:

• зрители запомнили именно соло
• зрители запомнили именно это соло

These are possible, but именно его is elegant and natural.

Именно adds emphasis. It means something like:

• exactly
• precisely
• specifically
• the very one

So:

зрители запомнили его = the audience remembered it
зрители запомнили именно его = the audience remembered that one specifically

It creates contrast. The sentence is not just saying the solo was remembered; it is saying that despite being short, that was the thing the audience remembered.

Yes, the basic meaning would stay similar, but the focus would change.

The original:

Хотя соло было коротким, зрители запомнили именно его.

This puts the emphasis naturally at the end on именно его.

Other possible orders:

• Хотя соло было коротким, именно его зрители запомнили.
  Stronger emphasis on именно его right away.

• Именно его зрители запомнили, хотя соло было коротким.
  Very strong contrast and emphasis on it specifically.

Russian word order is flexible, but it is not random: it helps show what is new, important, or contrastive. In the original sentence, the end position makes его especially prominent.

Зрители is nominative plural. It is the subject of запомнили.

So the main clause is:

• зрители = the spectators / the audience members
• запомнили = remembered
• именно его = that specifically / it in particular

Literally:

the spectators remembered exactly it

More natural English:

the audience remembered that specifically or it was the solo they remembered