Nella corsia accanto nuota una ragazza molto veloce.

Nella is the contraction of in + la.

• in = in
• la = the

So nella corsia means in the lane.

Italian very often combines a preposition with a definite article:

• in + il = nel
• in + lo = nello
• in + la = nella
• in + i = nei
• in + gli = negli
• in + le = nelle

So nella is just the normal combined form here.

Corsia usually means lane. The exact kind of lane depends on context.

It can mean:

• a traffic lane on a road
• an aisle or lane in a larger space
• a swimming lane in a pool

In this sentence, because of nuota (swims), corsia is understood as a swimming lane.

Accanto means next to, beside, or adjacent.

In la corsia accanto, it means the lane next to it / the adjacent lane.

Italian often places words like this after the noun:

• la porta accanto = the door next to it / the next door
• la stanza accanto = the room next door
• la corsia accanto = the lane next to it

So this is a natural way to say the adjacent lane.

Sometimes you will also see accanto a when the thing being referred to is stated explicitly:

• la corsia accanto alla nostra = the lane next to ours
• si siede accanto a me = he/she sits next to me

Here, the reference is left implicit.

Both word orders are possible in Italian.

The basic order is:

• Una ragazza nuota nella corsia accanto.

But Italian is much more flexible than English. In this sentence, the speaker puts the location first:

• Nella corsia accanto = in the next lane

Then comes the verb:

• nuota

And then the subject:

• una ragazza molto veloce

This kind of order is very common when the speaker first sets the scene and then introduces who is there or what is happening.

So Nella corsia accanto nuota una ragazza molto veloce sounds natural and a bit more descriptive or narrative than the more neutral Una ragazza molto veloce nuota nella corsia accanto.

Una ragazza means a girl, so it introduces someone not yet identified.

Italian often uses the indefinite article when bringing in new information, especially after the verb:

• C'è un uomo fuori. = There is a man outside.
• Arriva una macchina rossa. = A red car is arriving.
• Nuota una ragazza molto veloce. = A very fast girl is swimming.

If the speaker meant a specific, already known girl, la ragazza could be used instead:

• Nella corsia accanto nuota la ragazza molto veloce.

But that would usually imply the listener already knows which girl is being referred to.

Because molto here is an adverb, not an adjective.

It modifies veloce:

• molto veloce = very fast

When molto means very, it does not change form:

• molto veloce
• molto interessante
• molto bella

But when molto is an adjective meaning much / many, it does agree:

• molta acqua = a lot of water
• molti libri = many books
• molte ragazze = many girls

So in this sentence, molto stays unchanged because it means very.

Because veloce describes the girl, not the action directly.

• una ragazza molto veloce = a very fast girl

Here, veloce is an adjective modifying ragazza.

If you wanted to describe how she swims, you could use an adverb:

• nuota molto velocemente = she swims very quickly

So compare:

• una ragazza molto veloce = a very fast girl
• nuota molto velocemente = she swims very quickly

Both are possible, but they focus on slightly different things.

It does match ragazza, but veloce is one of those adjectives that has the same form for masculine and feminine singular.

• un ragazzo veloce = a fast boy
• una ragazza veloce = a fast girl

The plural changes:

• ragazzi veloci
• ragazze veloci

So the agreement is there, but you only see it clearly in the plural.

Italian present tense can do both, just like English sometimes can depending on context.

Nuota can mean:

• she swims
• she is swimming

In this sentence, because it describes what is happening in a nearby lane, the most natural interpretation is she is swimming right now.

Italian often uses the simple present where English prefers the present continuous.

Italian also has a continuous form:

• sta nuotando = is swimming

So you could also say:

• Nella corsia accanto sta nuotando una ragazza molto veloce.

That version makes the ongoing action more explicit, but the original sentence is perfectly normal.

Yes, absolutely.

That version is grammatically correct and probably feels more straightforward to an English speaker because it follows a more familiar subject + verb + place order:

• Una ragazza molto veloce nuota nella corsia accanto.

The original sentence:

• Nella corsia accanto nuota una ragazza molto veloce.

puts more focus on the location first. Italian uses word order flexibly to highlight different parts of the message.

So the difference is mostly about focus and style, not basic meaning.

They are similar, but not always identical.

• accanto = next to, right beside
• vicino = near, close

So:

• la corsia accanto = the adjacent lane / the lane right next to it
• la corsia vicina = the nearby lane / the close lane

In many situations they are close in meaning, but accanto usually suggests more direct adjacency.

Yes, it is natural.

A native speaker might also say:

• Nella corsia accanto c'è una ragazza molto veloce che nuota.
• Una ragazza molto veloce nuota nella corsia accanto.
• Nella corsia di fianco nuota una ragazza molto veloce.

But the original sentence is perfectly normal. It sounds like a descriptive sentence, especially in narration or spoken observation, where the speaker first points out the place and then says who is there.