Как только дирижёр поднял руки, хор запел.

Here как только is a fixed expression meaning as soon as.

So:

• Как только дирижёр поднял руки... = As soon as the conductor raised his hands...

Even though как by itself often means how and только often means only, in this sentence you should learn как только as one conjunction, not translate the words separately.

Russian uses a comma because Как только дирижёр поднял руки is a subordinate clause, and хор запел is the main clause.

So the structure is:

• Как только
  • subordinate clause, main clause

This is very normal in Russian:

• Как только он пришёл, мы начали ужинать.
• Когда урок закончился, все ушли.

If you reverse the order, the comma still stays:

• Хор запел, как только дирижёр поднял руки.

They are both past tense masculine singular forms.

In Russian past tense, the verb agrees with the subject in gender and number:

• masculine: -л
• feminine: -ла
• neuter: -ло
• plural: -ли

So:

• дирижёр поднял = the conductor raised
• хор запел = the choir began to sing

Both дирижёр and хор are masculine singular nouns, so the verbs are masculine singular too.

This is about aspect.

• поднял is perfective
• поднимал is imperfective

In this sentence, the meaning is a single completed action: the conductor raised his hands, and then the choir started singing. That is why поднял fits naturally.

Compare:

• Как только дирижёр поднял руки, хор запел.
  As soon as the conductor raised his hands, the choir began to sing.

• Дирижёр поднимал руки, а хор ждал.
  The conductor was raising his hands, and the choir was waiting.

So поднял presents the action as one complete event.

Because запел emphasizes the beginning of the singing.

• пел = was singing / sang
• запел = started singing / burst into song

In this sentence, that beginning is exactly the point:

• first, the conductor raised his hands
• then, the choir started singing

So запел is more precise than пел here.

Руки is in the accusative plural because it is the direct object of поднял.

The basic form is:

• рука = hand
• plural nominative: руки
• plural accusative: руки

For inanimate nouns in Russian, the accusative plural is usually the same as the nominative plural. Body parts like руки behave grammatically as inanimate nouns.

So:

• поднять руки = to raise one’s hands

Because Russian often leaves out possessive words when the owner is obvious from context.

So:

• дирижёр поднял руки naturally means the conductor raised his hands

Adding свои is possible:

• дирижёр поднял свои руки

But in a normal sentence, that often sounds unnecessary unless you want extra emphasis or contrast.

Russian does this a lot with body parts and personal belongings when the meaning is clear.

Because хор is grammatically a singular noun in Russian.

It is a collective group, but grammatically it behaves like one unit:

• хор запел = the choir began to sing

So the verb is singular too.

This is similar to English, where the choir sang also usually takes a singular verb in standard usage.

Russian has no articles.

So Russian does not have separate words corresponding to English a/an and the. Whether something is definite or indefinite is understood from context.

That means:

• дирижёр can mean a conductor or the conductor
• хор can mean a choir or the choir

In this sentence, the context makes them definite in English: the conductor, the choir.

Yes. Russian word order is fairly flexible, though the neutral version here is very natural:

• Как только дирижёр поднял руки, хор запел.

You could also say:

• Хор запел, как только дирижёр поднял руки.

The basic meaning stays the same, but the emphasis changes a little. The original sentence puts the earlier action first and feels very natural for a sequence of events.

Russian word order often reflects focus and style, not just grammar.

Because the correct pronunciation is with ё: дирижёр.

The letter ё is pronounced roughly like yo in york or your, depending on accent and comparison. It also shows that the stress is on that syllable:

• ди-ри-ЖЁР

In many printed texts, Russian often writes е instead of ё, so you may sometimes see дирижер, but the pronunciation is still дирижёр.

Yes. Как только is not limited to the past tense. It means as soon as and can be used whenever one action happens immediately after another.

For example:

• Как только он придёт, мы начнём.
  As soon as he arrives, we’ll begin.

• Как только закончится дождь, пойдём гулять.
  As soon as the rain stops, we’ll go for a walk.

So in your sentence it happens to introduce a past action, but the expression itself is more general.