К вечеру слабость стала меньше, и я наконец доел суп, который остался после обеда.

К вечеру means by evening or toward evening. It shows a change that happened gradually and was true by that time.

• К вечеру слабость стала меньше = By evening, the weakness had lessened
• Вечером usually just means in the evening

So here к вечеру fits better because the sentence is about the condition improving over time.

This literally means the weakness became less.

A few things are happening here:

• слабость = weakness
• стала = became
• меньше = less

Russian often says became less/more where English might prefer got better, lessened, or was weaker.

So:

• слабость стала меньше = literally the weakness became less
• natural English: the weakness lessened / I felt less weak

Because the subject is слабость, and слабость is a feminine noun in Russian.

Past tense verbs agree with the subject in gender and number:

• стал = masculine
• стала = feminine
• стало = neuter
• стали = plural

So:

• слабость стала = the weakness became

Меньше is the comparative form meaning less or smaller.

In this sentence it does not change for gender or number. That is normal. Comparatives like меньше, больше, лучше, хуже are usually invariable.

So even though слабость is feminine, you still say:

• слабость стала меньше not
• слабость стала меньшая

Those are different structures.

Доел comes from доесть, which means to finish eating something that was already partly eaten.

That is a very natural choice here because the soup was left over from lunch.

Compare:

• ел суп = was eating / ate soup (process, not focused on completion)
• съел суп = ate the soup up (finished it)
• доел суп = finished eating the remaining soup

So доел strongly suggests there was some soup left, and the speaker finished the rest of it.

Because past tense verbs with я show the speaker’s gender.

• я доел = the speaker is male
• я доела = the speaker is female

So this sentence is spoken by a man, or at least the speaker is being presented as male.

Наконец means finally or at last.

It often suggests that something took time, was difficult, or happened only after a delay.

In this sentence:

• и я наконец доел суп = and I finally finished the soup

That fits the context well: the speaker was weak, then by evening felt better, and only then managed to finish the soup.

Because и is joining two full clauses, each with its own subject and predicate:

1. К вечеру слабость стала меньше
2. я наконец доел суп, который остался после обеда

In Russian, when и connects two independent clauses, a comma is usually needed.

Because который остался после обеда is a relative clause describing суп.

• суп = the soup
• который остался после обеда = which was left after lunch

Russian normally separates this kind of descriptive subordinate clause with a comma.

Который is the relative pronoun meaning which/that.

It refers back to суп, so it matches суп in:

• gender: masculine
• number: singular

Inside its own clause, который is the subject of остался, so it is in the nominative case.

So:

• суп, который остался... = the soup which remained...

Остался is the past tense of остаться, meaning to remain / to be left.

It is masculine singular because it agrees with суп:

• суп остался = the soup remained / was left

So the relative clause literally says:

• which remained after lunch or more naturally,
• which was left over after lunch

The preposition после always takes the genitive case.

So:

• обед = nominative
• после обеда = genitive, after lunch

This is a fixed rule:

• после работы = after work
• после урока = after the lesson
• после обеда = after lunch

Usually обед means the main daytime meal, so in many contexts it is best translated as lunch.

Depending on culture, schedule, or context, meal words do not always match English perfectly. But in a sentence like this, после обеда will most naturally be understood as after lunch.

Yes, Russian word order is flexible, but the given order sounds natural.

• К вечеру puts the time frame first
• слабость стала меньше gives the change in condition
• и я наконец доел суп moves to the speaker’s action
• который остался после обеда adds extra information about the soup

A more rigid English-like order is often possible in Russian, but this version flows well and sounds normal.

Both are natural, but they focus on different things.

• слабость стала меньше focuses on the symptom itself: the weakness lessened
• я стал чувствовать себя лучше focuses on the person’s overall condition: I started to feel better

The original sentence is a bit more precise and descriptive because it specifically mentions weakness.