Затопление началось в подвале.

Because the verb is начаться — the perfective, intransitive verb meaning to begin / to start.

In the past tense, Russian verbs agree with the subject in gender and number. The subject here is затопление, which is neuter singular, so the past-tense form is:

• masculine: начался
• feminine: началась
• neuter: началось
• plural: начались

The -сь / -ся part is the reflexive ending that belongs to the verb начаться.

So:

• затопление началось = the flooding began

Because nouns ending in -ие are usually neuter in Russian.

So затопление is a neuter noun, and that is why the verb is also in the neuter past form началось.

A learner can often use the ending as a clue:

• -ие → often neuter
• -а / -я → often feminine
• consonant ending → often masculine

It is in the nominative case because it is the subject of the sentence.

In this sentence, the thing that began is затопление. So Russian uses the nominative:

• Затопление началось = The flooding began

If you changed the sentence so that flooding became the object, the case would probably change.

Because в подвале expresses location: in the basement.

With в, Russian often uses:

• accusative for motion into somewhere
• prepositional for being in somewhere

So:

• в подвал = into the basement
• в подвале = in the basement

Here the sentence says where the flooding began, not that something moved into the basement. So в подвале is correct.

It is the prepositional case.

The basic noun is подвал. After в with the meaning of location, it becomes:

• подвал → в подвале

This is a very common pattern:

• в доме = in the house
• в городе = in the city
• в подвале = in the basement

Because началось is perfective, and it focuses on the fact that the event started.

• началось = started / began as a completed event of starting
• начиналось = was starting / used to start / started (in a more process-focused or background sense)

In a simple statement like The flooding began in the basement, Russian normally prefers началось.

So this sentence presents the beginning as a single event.

Yes. Russian word order is flexible.

The neutral order here is:

• Затопление началось в подвале.

But you could also say:

• В подвале началось затопление.

That version puts more focus on where it began: In the basement, the flooding began.

The basic meaning stays the same, but the emphasis changes.

Not exactly.

Both can relate to flooding, but they are not always interchangeable.

• затопление often means flooding/inundation, especially the act or result of something being flooded
• наводнение more often means a flood as a natural disaster, especially large-scale flooding

So in a building or basement context, затопление sounds very natural.

For example:

• затопление подвала = flooding of the basement
• наводнение в городе = flood in the city

Russian has no articles, so there is nothing to translate directly as the or a.

So в подвале could mean:

• in the basement
• in a basement

Which one is intended depends on context.

English requires an article, but Russian does not.

Because that would mean something different.

• Затопление началось в подвале. = The flooding began in the basement.
• Подвал был затоплен. = The basement was flooded.

The first sentence focuses on the beginning of the event. The second focuses on the state/result affecting the basement.

So the Russian sentence is not describing the basement as already flooded; it is describing where the flooding started.

The stress is:

• затопле́ние
• начало́сь
• подва́ле

So roughly:

• za-ta-ple-ni-ye na-cha-LOS' v pad-VA-le

A natural pronunciation is:

Затопле́ние начало́сь в подва́ле.

Stress matters in Russian, so it is worth learning words together with their stress.

In this sentence, no — затопление is clearly a noun.

It is a verbal noun, something like flooding in English. English also uses flooding as a noun:

• Flooding began in the basement.

So the structure is:

• затопление = noun, subject
• началось = verb, past tense
• в подвале = location

That is why the sentence is built more like The flooding began..., not Something flooded...