Перед концертом дирижёр долго выбирал репертуар и убрал одну слишком сложную песню.

Because перед normally requires the instrumental case when it means before / in front of.

So:

• концерт = concert
• перед концертом = before the concert

This is a very common pattern:

• перед уроком = before the lesson
• перед встречей = before the meeting
• перед ужином = before dinner

In this sentence, перед концертом means before the concert in a time sense.

It is changing into the instrumental singular form.

The noun концерт is a masculine noun, and many masculine nouns in the instrumental singular take -ом:

• концерт → концертом
• стол → столом
• директор → директором

So the ending -ом here shows the case required by перед.

Дирижёр means conductor. The letter ё is important because it shows both the pronunciation and the stress.

• дирижёр is pronounced with stress on the last syllable
• ё is always stressed

So this word is pronounced roughly dee-ree-ZHYOR.

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

Because выбирал is imperfective, and it emphasizes the process of choosing.

• выбирать / выбирать (imperfective) = to be choosing, to choose over a period of time
• выбрать (perfective) = to choose, to make the final choice

Here we have долго выбирал = was choosing for a long time. That sounds natural because the sentence focuses on the duration of the activity.

If you said долго выбрал, that would sound wrong, because perfective verbs usually do not combine naturally with долго when you mean an ongoing process.

Because here the speaker is talking about a completed action: he removed one song from the repertoire.

• убирать (imperfective) = to be removing / to remove repeatedly
• убрать (perfective) = to remove, take away, eliminate completely

So the contrast is very natural:

• долго выбирал = he spent a long time choosing
• убрал одну песню = he removed one song

The first verb describes a process, and the second describes a finished result.

Not exactly. Убрать can mean several things depending on context:

• to tidy up / put away
• to remove
• to take out
• sometimes to eliminate from a list, plan, or schedule

In this sentence, убрал одну слишком сложную песню means he removed one overly difficult song from the repertoire. It does not mean he physically cleaned anything.

Because песню is the direct object of убрал, so it must be in the accusative case. The numeral/adjective одна must agree with it.

Feminine singular forms:

• nominative: одна сложная песня
• accusative: одну сложную песню

That is why all three words change:

• одна → одну
• сложная → сложную
• песня → песню

Because adjectives must agree with the noun they describe in gender, number, and case.

Here the noun is:

• песню = feminine singular accusative

So the adjective must also be feminine singular accusative:

• сложная песня = a difficult song
• сложную песню = a difficult song (as an object)

This agreement is one of the core features of Russian grammar.

No. Слишком means too, not just very.

• очень сложную = very difficult
• слишком сложную = too difficult

So одну слишком сложную песню means one song that was excessively difficult, probably too difficult to include in the concert.

Russian does not have articles like a or the.

So:

• репертуар can mean repertoire, the repertoire, or sometimes a repertoire, depending on context
• песню can mean a song or the song

English uses articles to show whether something is specific or general. Russian usually leaves that to context, word order, and surrounding information.

In this sentence, одну ... песню clearly means one song, and the context makes it natural in English to say one overly difficult song.

Because репертуар is the object of выбирал, and with this verb it appears in the accusative case. For many inanimate masculine nouns, the accusative singular is the same as the nominative singular.

So:

• nominative: репертуар
• accusative: репертуар

That is why the form does not visibly change.

Compare:

• Я вижу стол. = I see the table.
  (стол stays the same)
• Я вижу машину. = I see the car.
  (машина → машину)

Долго means for a long time.

It modifies выбирал:

• долго выбирал = chose for a long time / was choosing for a long time

This is another clue that выбирал is imperfective, because долго often goes naturally with actions seen as ongoing processes.

Yes. Russian word order is more flexible than English word order, because case endings show grammatical relationships.

The original sentence is neutral and natural:

• Перед концертом дирижёр долго выбирал репертуар и убрал одну слишком сложную песню.

But other orders are possible, for example to emphasize different parts:

• Дирижёр перед концертом долго выбирал репертуар...
• Одну слишком сложную песню дирижёр убрал перед концертом.

Even though word order can change, the original version sounds straightforward and natural for narration.

Песня specifically means song. It works if the repertoire includes songs.

But depending on context, other nouns might also be possible:

• произведение = piece / work
• номер = number, performance item
• композиция = composition, musical piece

So песня is correct if the text really means song, but in a classical concert context, learners might expect a word like произведение. That is more a question of vocabulary choice than grammar.