Когда пианист ошибся, учительница попросила его снова посмотреть в ноты и не терять такт.

Ошибся is the perfective past form, so it refers to one completed mistake at a specific moment.

In this sentence, the idea is:

• the pianist made a mistake,
• then the teacher reacted.

That sequence fits perfective ошибся very well.

If you used ошибался, that would be imperfective, suggesting something like:

• he was making mistakes,
• he kept making mistakes,
• or the focus is on the process rather than one single slip.

So Когда пианист ошибся... means When the pianist made a mistake...

This is a very common Russian pattern:

• попросить + someone (accusative) + infinitive

So:

• попросила = asked
• его = him
• посмотреть = to look

Together, попросила его посмотреть means asked him to look.

Russian often uses this direct structure where English uses asked him to...

Because with попросить in this pattern, the person being asked is normally in the accusative case, not the dative.

So:

• попросила его = asked him
• not попросила ему

For masculine animate nouns and pronouns, the accusative often looks like the genitive, which is why you see его.

Compare:

• Я попросил брата помочь. = I asked my brother to help.
• Она попросила его подождать. = She asked him to wait.

Посмотреть is perfective, and here it means to take a look / to look again once.

The teacher is asking for a single concrete action:

• снова посмотреть в ноты = to look at the sheet music again

If you used смотреть, it would sound more like an ongoing action: to be looking or to look for some time.

So the aspect choice matches the meaning:

• посмотреть = one deliberate look
• смотреть = the process of looking

In Russian, смотреть в ноты is a normal expression meaning to look at the sheet music.

More generally, смотреть в + accusative is often used when looking into or at something like a text, mirror, window, or music:

• смотреть в книгу
• смотреть в зеркало
• смотреть в окно
• смотреть в ноты

English usually just says look at, but Russian often chooses a different preposition depending on the object.

So в ноты is idiomatic here.

Because ноты usually means sheet music / musical notes as a plural noun in Russian.

Even when English might say the music or the score, Russian often says ноты.

Singular нота usually means a note as an individual musical note, while ноты often means the written music in general.

So:

• посмотреть в ноты = look at the sheet music

This is about aspect under negation.

Here не терять такт uses the imperfective verb терять, which sounds natural for a warning or instruction meaning:

• don’t lose the beat
• keep the rhythm steadily
• don’t let the pulse slip

The imperfective is common when Russian expresses a general prohibition or ongoing control.

Не потерять такт is possible in some contexts, but it would sound more like not to lose it at one particular moment. In this sentence, the teacher’s advice is broader and more continuous, so не терять works better.

In this musical context, такт refers to the beat, meter, or sense of rhythmic timing.

So не терять такт means something like:

• don’t lose the beat
• keep time
• don’t fall out of the rhythm

Be careful: такт can also mean tact or social sensitivity in other contexts, but here the musical meaning is clearly intended.

Because Когда пианист ошибся is a subordinate clause introduced by когда.

Russian separates that clause from the main clause with a comma:

• Когда пианист ошибся, учительница попросила...

This is like English:

• When the pianist made a mistake, the teacher asked...

Russian punctuation is quite strict about this kind of clause.

Because посмотреть в ноты and не терять такт are two infinitives that both depend on the same verb, попросила.

The teacher asked him:

1. снова посмотреть в ноты
2. не терять такт

These are two coordinated actions inside the same request, so no extra comma is needed before и.

Not completely. Russian word order is fairly flexible, but the given order is natural and neutral.

• попросила его снова посмотреть в ноты sounds normal
• снова is placed before посмотреть because it modifies that action: look again

You can move parts around for emphasis, but the meaning or tone may shift slightly. For example:

• Когда пианист ошибся, учительница попросила его посмотреть в ноты снова...

This is possible, but less natural in ordinary speech than снова посмотреть.

So the sentence uses a very standard, natural order.

Yes. Russian has two common ways to express this idea:

1. попросила его посмотреть...
2. попросила, чтобы он посмотрел...

Both can mean asked him to look...

The version in your sentence, попросила его посмотреть, is more compact and very common when the subject of the infinitive is clear.

The чтобы version can sound a bit more explicit or heavier:

• Учительница попросила, чтобы он снова посмотрел в ноты и не терял такт.

So the original sentence uses the shorter and very natural infinitive construction.