Не успел дирижёр поднять руки, как хор запел.

This is a very common Russian pattern meaning no sooner had X happened than Y happened or before X could happen, Y already happened.

So:

Не успел дирижёр поднять руки, как хор запел.
= The conductor had barely raised his hands when the choir began to sing.

Literally, не успел means did not manage / did not have time, but in this construction it often expresses that the second action happened almost immediately after the first one started or was about to happen.

A rough English equivalent is:

• No sooner had the conductor raised his hands than the choir started singing.
• The conductor had barely raised his hands when the choir began to sing.

Because it agrees with дирижёр, which is masculine singular.

• дирижёр = conductor
• masculine singular subject
• so the past-tense verb is успел

Compare:

• Не успела певица выйти, как начались аплодисменты.
  The singer had barely come out when the applause began.
• Не успели музыканты сесть, как дирижёр начал.
  The musicians had barely sat down when the conductor began.

Russian past tense changes for gender and number:

• успел — masculine
• успела — feminine
• успело — neuter
• успели — plural

Because поднять is perfective, and after успеть Russian usually uses the infinitive that refers to completing a single action.

• поднять = to raise, to lift up once/completely
• поднимать = to be raising / to raise repeatedly / ongoing process

In this sentence, the idea is that the conductor barely got to the point of raising his hands before the choir started. That fits the perfective поднять.

So:

• успел поднять = managed to raise
• успел поднимать would sound odd here, because managed to be raising is not what is meant

Because a conductor normally raises both hands.

Russian often uses the plural body-part noun in contexts like this:

• поднять руки = raise one’s hands / arms
• опустить глаза = lower one’s eyes
• закрыть глаза = close one’s eyes

English often uses possessives like his hands, but Russian often does not need them when the owner is obvious from context.

So поднять руки naturally means raise his hands here.

Руки is in the accusative plural.

The verb поднять takes a direct object:

• поднять что? — руки

For the noun рука:

• nominative singular: рука
• nominative plural: руки
• accusative plural: руки

Since рука is an inanimate noun, its accusative plural looks the same as the nominative plural.

So here:

• поднять руки = to raise hands

Russian often leaves out possessives when the meaning is obvious.

In English, we usually say:

• He raised his hands

In Russian, it is very normal to say simply:

• Он поднял руки

because everyone understands that they are his hands.

You could say свои руки, but it is usually unnecessary unless you want emphasis or contrast:

• Он поднял свои руки, а не чужие.
  He raised his own hands, not someone else’s.

So in your sentence, omitting the possessive is the most natural choice.

Because this sentence has two clauses:

1. Не успел дирижёр поднять руки
2. как хор запел

The word как here introduces the second clause in the fixed construction не успел ..., как ...

So the comma separates the two parts of the sentence.

This is standard punctuation in Russian for this pattern.

No. Here как does not mean how.

In this construction, как is closer to:

• when
• than
• and then suddenly

depending on how you translate it.

In English, the whole pattern is usually translated idiomatically rather than word-for-word:

• No sooner had the conductor raised his hands than the choir began to sing.
• The conductor had barely raised his hands when the choir started singing.

So you should learn не успел ..., как ... as a set expression.

Because запел is perfective and means began to sing.

• пел = was singing / sang / used to sing
• запел = started singing

The sentence describes the start of a new action, so запел is the natural choice.

This also matches the suddenness of the whole sentence:

• the conductor barely raised his hands,
• and immediately the choir started singing.

If you said хор пел, the meaning would shift more toward the choir was singing, not specifically began singing.

Because хор means choir, and grammatically it is a singular noun in Russian, just like in English.

So the verb is singular too:

• хор запел = the choir began to sing

Even though a choir consists of many people, Russian treats хор as one collective unit here.

Yes, Russian word order is flexible, though the original version sounds very natural.

Original:

Не успел дирижёр поднять руки, как хор запел.

You might also hear:

• Дирижёр не успел поднять руки, как хор запел.

Both are understandable. The original puts не успел first, which emphasizes the rapid sequence of events right away.

Russian often uses word order to highlight focus rather than to mark basic grammar.

It is completely normal Russian and can appear in both speech and writing, though the pattern не успел ..., как ... often feels a bit more polished or narrative than the simplest everyday phrasing.

A more conversational alternative might be:

• Дирижёр только поднял руки, и хор сразу запел.
  The conductor had just raised his hands, and the choir immediately started singing.

But your original sentence is very natural and elegant.

The word is дирижёр, with stress on the last syllable:

• ди-ри-жёр

The letter ё always carries stress. It is important because it tells you both the pronunciation and the stressed syllable.

In many printed texts, ё is sometimes written as е, so you may see дирижер, but it is still pronounced дирижёр.

For learners, it is very useful to remember the correct spelling with ё whenever possible.