После ремонта комната стала ярче.

Because после requires the genitive case.

So:

• ремонт = dictionary form, nominative
• ремонта = genitive singular

After после, Russian uses genitive to mean after:

• после урока = after the lesson
• после работы = after work
• после ремонта = after the repair / renovation

So ремонта is there because of the preposition после.

Here после means after.

In this sentence, it means after the repair/renovation was finished. It often refers to time, but that time can be connected to an event:

• после завтрака = after breakfast
• после фильма = after the movie
• после ремонта = after the renovation

So it does not have to be a clock time; it can be after an event or process.

Because комната is the subject of the sentence — the thing that became brighter.

In Russian, subjects are normally in the nominative case. Here:

• комната = the room
• стала = became
• ярче = brighter

So комната stays in the nominative because it is doing the grammatical job of the subject.

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

Since комната is:

• singular
• feminine

the verb must be стала.

Compare:

• комната стала = the room became
• зал стал = the hall became
• помещение стало = the помещение became
• комнаты стали = the rooms became

This is a very common feature of Russian past tense.

Because стать means to become, while быть means to be.

This sentence describes a change of state:

• before the renovation, the room was one way
• after the renovation, it became brighter

So:

• комната стала ярче = the room became brighter
• комната была ярче = the room was brighter

The second one does not naturally express change; it just describes how the room was at some past time.

Because ярче is the comparative form: brighter.

The sentence is not just saying the room is bright; it is saying the room is more bright than before.

So:

• яркая = bright
• ярче = brighter

After стать, Russian very often uses the comparative to show change:

• стало лучше = became better
• стало теплее = became warmer
• стала ярче = became brighter

If you used яркая, that would not fit this structure.

Ярче is the comparative form of яркий.

The basic adjective is:

• яркий = bright

The comparative is:

• ярче = brighter

This is a normal type of Russian comparative, but the consonant changes:

• к → ч

So:

• яркий → ярче

Russian comparatives are not always formed in one single predictable way, so it is good to learn them as vocabulary patterns. Other common examples:

• громкий → громче
• тихий → тише
• большой → больше

Yes. Russian often uses the comparative without naming the second thing.

In this sentence, the comparison is understood from context:

• brighter than before the renovation
• brighter than it used to be

This is very natural in Russian. English does the same:

• It got better
• The room became brighter

Even if we do not say exactly than what, the listener understands it from context.

Yes, that is grammatically correct.

But there is a difference in style:

• стала ярче = more natural, shorter, very common
• стала более яркой = also correct, but heavier and less direct

In стала более яркой, the adjective is in the instrumental case because full predicate adjectives after стать are commonly put in instrumental:

• стала яркой
• стала более яркой

So both work, but стала ярче is the more natural everyday version here.

Yes. Russian word order is flexible.

You can also say:

• Комната стала ярче после ремонта.

Both versions are correct. The difference is mainly in focus:

• После ремонта комната стала ярче.
  This starts with the time/context: after the renovation
• Комната стала ярче после ремонта.
  This starts with the subject: the room

The original version sounds very natural because it sets up the context first.

It can mean different things depending on context.

Ремонт is a broad word that can mean:

• repair
• renovation
• redecoration
• remodeling work

With a room, English often translates it more naturally as renovation or redecoration, because the sentence suggests the room’s appearance changed. But the Russian word itself is broader than just one exact English word.