Шпинат быстро сварился, а сельдерей сделал вкус ярче.

Because both are the subjects of their clauses, so they are in the nominative case:

• Шпинат быстро сварился = spinach cooked quickly
• сельдерей сделал вкус ярче = celery made the taste brighter

In Russian, the subject is normally in the nominative, just like in English we use I/he/she rather than me/him/her for subjects.

Сварился is the past tense of свариться, which means to get cooked by boiling, to boil until done, or simply to cook through in this context.

So шпинат быстро сварился means something like:

• The spinach cooked quickly
• The spinach boiled quickly
• The spinach became cooked quickly

It focuses on the spinach reaching the cooked state.

The -ся makes the verb reflexive, but in many cases that does not mean a literal English reflexive like washed itself. Here it is part of the normal verb свариться.

Compare:

• сварить что-то = to boil/cook something
• свариться = to boil/cook through, become boiled/cooked

So:

• Я сварил шпинат = I cooked the spinach
• Шпинат сварился = The spinach cooked / got cooked

In this sentence, the reflexive form is used because the spinach is presented as the thing that ended up cooked, not as the direct object of an explicit cook.

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

• шпинат is masculine singular, so: сварился
• сельдерей is also masculine singular, so: сделал

Compare:

• masculine: он сделал
• feminine: она сделала
• neuter: оно сделало
• plural: они сделали

So the verb endings tell you something about the noun’s gender.

А often links two clauses while showing a contrast or switch of focus.

Here the sentence presents two related facts:

• the spinach cooked quickly
• the celery made the taste brighter

So а works like:

• while
• whereas
• and meanwhile
• sometimes a mild but

It is softer than но, which usually gives a stronger but.
And it is more contrastive than и, which would simply mean and.

It is in the accusative case, because it is the direct object of сделал:

• сделал что? → вкус

For many masculine inanimate nouns, the accusative looks exactly like the nominative, so:

• nominative: вкус
• accusative: вкус

That is why the form does not change.

Ярче is the comparative form of яркий, meaning brighter or more vivid.

Russian very often uses this simple comparative form after verbs like сделать:

• сделать лучше = to make better
• сделать чище = to make cleaner
• сделать ярче = to make brighter / more vivid

So сделал вкус ярче literally means made the taste brighter.

You could build a longer phrase with более..., but in ordinary Russian the simple comparative ярче is the natural choice here.

This is a very common Russian pattern:

• сделать + object + comparative

Examples:

• сделать суп вкуснее = make the soup tastier
• сделать комнату светлее = make the room lighter
• сделать голос тише = make the voice quieter

So сельдерей сделал вкус ярче follows a normal Russian structure. Even if English might phrase it a bit differently, the Russian grammar is straightforward.

Быстро is an adverb meaning quickly. It modifies сварился:

• сварился = cooked
• быстро сварился = cooked quickly

Adverbs in Russian do not change form for gender, number, or case, so быстро stays the same.

Both verbs describe completed results:

• сварился = the spinach ended up cooked
• сделал = the celery had the effect of making the taste brighter

This is why perfective verbs are natural here:

• свариться is perfective
• сделать is perfective

If you used imperfective forms, the focus would shift more toward process or repeated action rather than the finished result.

Yes. Russian word order is flexible, although the original version sounds natural and neutral:

• Шпинат быстро сварился, а сельдерей сделал вкус ярче.

You could rearrange parts for emphasis, for example:

• Быстро сварился шпинат, а сельдерей сделал вкус ярче.
• Шпинат сварился быстро, а сельдерей сделал вкус ярче.

These are still understandable, but the original order is the most neutral and textbook-like. Russian often moves words around to highlight new information, contrast, or rhythm.