Барабан задавал ритм, а флейта делала мелодию легче.

Because а is joining two separate clauses here:

• Барабан задавал ритм
• а флейта делала мелодию легче

Each part has its own subject and verb, so Russian normally puts a comma before а. This is very similar to using a comma before but in English, and often before and when two full clauses are joined.

А often adds a slight contrast or comparison, not just simple addition.

Here the idea is something like:

• the drum was doing one musical job,
• while the flute was doing a different one.

So а highlights that contrast of roles. In English, it may still be translated as and, but in Russian а suggests whereas / while / and on the other hand more than plain и would.

In the Russian past tense, verbs agree with the subject in gender and number.

• барабан is masculine singular, so: задавал
• флейта is feminine singular, so: делала

This is a very important feature of Russian past tense. Compare:

• он делал — masculine
• она делала — feminine
• оно делало — neuter
• они делали — plural

Both words are direct objects, so they are in the accusative case.

But accusative looks different depending on the noun type:

• ритм is masculine inanimate, and for masculine inanimate nouns, accusative singular usually looks the same as nominative singular:
  ритм → ритм
• мелодия is feminine, and its accusative singular changes:
  мелодия → мелодию

So the difference is not because the grammar is different; both are objects. It is just that Russian noun endings behave differently depending on gender and declension pattern.

Because легче is a comparative form, meaning lighter / easier / more light depending on context.

After verbs like делать, Russian often uses this pattern:

делать + object + comparative

For example:

• делать текст понятнее
• делать речь короче
• делать мелодию легче

The comparative легче does not change for gender, number, or case. That is why it stays легче, not лёгкую or лёгкой.

The imperfective aspect is used here because the sentence describes an ongoing scene or general musical function, not a single completed result.

So the idea is more like:

• the drum was setting / kept setting the rhythm
• the flute was making the melody lighter

If you used perfective forms such as задал or сделала, the sentence would focus more on a completed action or result at one moment. The imperfective is more natural for description, background, or process.

Yes, it is a very common collocation.

Задавать ритм means to set the rhythm, to establish the pace, or more broadly to drive the rhythm. It can be used:

• literally, in music
• figuratively, in other situations

For example:

• Барабаны задают ритм.
• Лидер задаёт ритм работе.

So this is a useful expression to remember as a chunk.

Yes. Russian word order is flexible because case endings and verb agreement already show the grammatical roles.

The sentence as written is neutral and natural:

Барабан задавал ритм, а флейта делала мелодию легче.

But you could change the order for emphasis, for example:

Ритм задавал барабан, а мелодию легче делала флейта.

That version puts extra focus on ритм and мелодию. So the meaning stays basically the same, but the emphasis changes.

Because Russian has no articles.

Russian does not have direct equivalents of English a/an and the. Whether something is definite or indefinite is usually understood from context.

So барабан, ритм, флейта, and мелодию can mean:

• a drum / the drum
• a rhythm / the rhythm
• a flute / the flute
• a melody / the melody

The surrounding context tells you which reading is intended.

Because the comparative of лёгкий is irregular-looking:

• лёгкий → легче

This involves a stem change:

• гк becomes гч / кч-like spelling in pronunciation
• ё changes to е

This is normal in Russian comparatives. Many common adjectives change form rather than just adding a simple ending. So легче is something you should learn as the comparative form of лёгкий.

Also, легче is pronounced roughly LYEKH-che.