После ремонта я вытерла пыль с плинтуса и поняла, что в комнате стало уютнее.

Because после always takes the genitive case.

• ремонт = nominative
• ремонта = genitive singular

So:

• после ремонта = after the repair / renovation

This is a fixed pattern:

• после урока = after the lesson
• после работы = after work
• после дождя = after the rain

Those are past tense feminine singular forms.

In Russian past tense, the verb agrees with the gender and number of the subject:

• я вытер = I wiped (male speaker)

• я вытерла = I wiped (female speaker)

• я понял = I understood/realized (male speaker)
• я поняла = I understood/realized (female speaker)

So this sentence tells us the speaker is female.

Russian often uses the perfective aspect when talking about completed actions in a sequence.

Here the speaker:

1. finished wiping the dust
2. then realized something

So perfective is very natural:

• вытерла = wiped up / finished wiping
• поняла = realized / came to understand

If you used imperfective forms like вытирала or понимала, the meaning would shift:

• вытирала = was wiping / used to wipe
• понимала = was understanding / understood in an ongoing sense

That would not fit this neat chain of completed events as well.

Yes, пыль is the direct object, so it is in the accusative case. But for this noun, the accusative looks the same as the nominative.

• nominative: пыль
• accusative: пыль

This happens because пыль is an inanimate feminine noun ending in -ь.

So in the sentence:

• я вытерла пыль = I wiped the dust

the case changed grammatically, even though the form did not change visibly.

Because с here means from / off, and with that meaning it normally takes the genitive case.

• плинтус = baseboard / skirting board
• плинтуса = genitive singular

So:

• вытерла пыль с плинтуса = wiped the dust off the baseboard

Compare:

• с пола = off the floor
• со стола = off the table
• с полки = off the shelf

Yes, in real life there are usually several sections of baseboard, and Russian can use either singular or plural depending on how the speaker imagines it.

Here the singular с плинтуса can sound like:

• from the baseboard as a surface in general
• from a particular section the speaker was dealing with

You could also hear:

• с плинтусов = off the baseboards

That would emphasize multiple baseboards more clearly. The singular here is not strange; it just presents the baseboard more as one continuous thing.

Что introduces a subordinate clause, like English that.

• я поняла, что... = I realized that...

So the structure is:

• main clause: я ... поняла
• subordinate clause: что в комнате стало уютнее

Russian often uses что after verbs of thinking, knowing, saying, realizing, noticing, etc.:

• я знаю, что... = I know that...
• она сказала, что... = she said that...
• мы поняли, что... = we realized that...

Because в meaning in for location takes the prepositional case.

• комната = room
• в комнате = in the room

This answers the question where?

Compare:

• в доме = in the house
• в школе = at school / in the school
• в машине = in the car

If there were movement into the room, Russian would use the accusative instead:

• в комнату = into the room

But here it is just location, so в комнате is correct.

Russian often uses an impersonal construction here:

• в комнате стало уютнее = it became cozier in the room / the room felt cozier

The verb стало is neuter singular, which is the default form in many impersonal expressions.

This structure focuses on the change in atmosphere or state, not on the room as a grammatical subject.

You could also say:

• комната стала уютнее

That is grammatical too, but it sounds a bit more like the room itself became cozier as a subject.
В комнате стало уютнее feels very natural when talking about the feeling of the space.

Уютнее is the comparative form of уютный / уютно.

In this sentence it behaves more like a predicative state word or an adverbial comparative:

• уютно = cozy
• уютнее = cozier / more cozy

After стало, Russian often uses this kind of form:

• стало лучше = it became better
• стало теплее = it became warmer
• стало тише = it became quieter
• стало уютнее = it became cozier

So even though English might make you expect an adjective agreeing with комната, Russian naturally says стало уютнее in this type of sentence.

Russian has no articles.

So nouns like ремонт, пыль, плинтус, and комната can mean:

• a repair / the repair
• dust / the dust
• a baseboard / the baseboard
• a room / the room

The exact meaning comes from context.

In this sentence, the context makes everything specific enough:

• после ремонта = after the renovation/repair that has just been talked about or is understood
• в комнате = in the room being discussed

Yes. Russian word order is fairly flexible, and changing it changes emphasis more than basic meaning.

The given sentence is neutral and natural:

• После ремонта я вытерла пыль с плинтуса и поняла, что в комнате стало уютнее.

Other possible orders:

• Я после ремонта вытерла пыль с плинтуса...
• Пыль с плинтуса я вытерла после ремонта...

These versions shift the focus:

• fronting после ремонта emphasizes the time frame
• fronting пыль с плинтуса emphasizes what exactly was wiped

So the original order works well as a smooth narrative sentence.