Завтра я спою песню на концерте.

Because спою is the perfective future form of спеть.

In Russian, there are two common ways to talk about the future:

• imperfective: буду петь = I will be singing / I will sing
  
  • focuses on the process, duration, or repeated action
• perfective: спою = I will sing / I will sing through / I will perform
  
  • focuses on the action as a completed whole

In this sentence, спою песню suggests I will sing a song (from beginning to end, as a performance), which is why perfective works very naturally.

So:

• Завтра я спою песню на концерте. = I’ll sing a song at the concert.
• Завтра я буду петь на концерте. = Tomorrow I’ll be singing at the concert.
  This sounds more like talking about the activity in progress, not necessarily one completed song.

The base form is спеть.

This is a perfective verb meaning to sing (through), to sing completely, to perform by singing.

A few useful related forms:

• спеть = infinitive
• спою = I will sing
• споёшь = you will sing
• споёт = he/she/it will sing

This verb is perfective, so it does not form the future with буду. Instead, its present-looking forms actually have future meaning.

Because it is the direct object of the verb, so it takes the accusative case.

The dictionary form is:

• песня = song

But after a transitive verb like спеть, the thing being sung is the direct object:

• спеть что? → песню

Since песня is a feminine noun ending in -я, its accusative singular changes to -ю:

• nominative: песня
• accusative: песню

So:

• Я спою песню. = I will sing a song.

Russian usually uses на with many events and public activities, where English often uses at:

• на концерте = at the concert
• на уроке = at/in class
• на встрече = at the meeting
• на работе = at work

So на концерте is the normal idiomatic choice for at a concert.

Using в концерте would sound unnatural in this meaning.

Because after на in this sentence, the noun is in the prepositional case.

The basic form is:

• концерт = concert

After на meaning at/on, we often use the prepositional:

• на концерте = at the concert

For this masculine noun, the prepositional singular ending is -е:

• nominative: концерт
• prepositional: концерте

Yes. Russian word order is fairly flexible, and changing it usually changes the emphasis, not the basic meaning.

The original sentence:

• Завтра я спою песню на концерте.

Other possible orders:

• Я завтра спою песню на концерте.
• На концерте я завтра спою песню.
• Песню я спою завтра на концерте.

These all mean roughly the same thing, but the focus shifts:

• Завтра first emphasizes tomorrow
• На концерте first emphasizes at the concert
• Песню first emphasizes the song

For a learner, the original version is a very natural and neutral one.

It can mean either, depending on context.

Russian has no articles like a or the, so:

• песню could mean a song
• or the song

The listener figures this out from context.

For example:

• if no specific song has been mentioned, it may sound like a song
• if both speakers already know which song is meant, it may mean the song

So Russian leaves this unspecified unless something else makes it clear.

It is pronounced спою́, with the stress on the last syllable.

A rough English approximation is:

• spa-YOO

More exactly:

• с = like s
• п = like p
• о in an unstressed position is reduced somewhat
• ю́ sounds like yoo, and it carries the stress

So the stress pattern is:

• спою́
• пе́сню
• на конце́рте

Stress is very important in Russian, so it is worth learning words together with their stress.

Because in Russian, the verb to be is usually not used in the present tense, and in this sentence it is not needed at all.

The sentence already has the main verb:

• спою = I will sing

So Russian simply says:

• Завтра я спою песню на концерте.

There is no need for an extra word like am or will be unless the grammar specifically calls for it.

Yes, you could, but the nuance is a little different.

• спою песню focuses on the song as a completed performance
• буду петь песню focuses more on the process of singing

So:

• Завтра я спою песню на концерте.
  = Tomorrow I’ll sing a song at the concert.
  This sounds like a complete, one-time performance.

• Завтра я буду петь песню на концерте.
  = Tomorrow I’ll be singing a song at the concert.
  This can sound more like describing what you’ll be doing.

In many situations both are possible, but спою is especially natural if you mean I will perform a song.

No. Here завтра is an adverb meaning tomorrow.

It does not change form here and does not depend on case in the way nouns do.

So in this sentence:

• Завтра = tomorrow
• я = I
• спою = will sing
• песню = song (accusative)
• на концерте = at the concert

Only the nouns песню and концерте are changing because of case. Завтра is just an adverb of time.