En la biblioteko oni legas trankvile.

Oni is an impersonal pronoun. It refers to people in general, not to one specific person. In English, depending on context, it can correspond to one, people, they, or even generic you.

So oni legas means something like people read, one reads, or reading is done in a general sense.

In Esperanto, verbs do not change according to person or number. The present tense always ends in -as:

• mi legas
• vi legas
• li legas
• ili legas
• oni legas

So legas is correct no matter who the subject is.

Because trankvile is an adverb, and it describes how the reading happens.

• trankvila = calm, quiet, peaceful (adjective)
• trankvile = calmly, quietly, peacefully (adverb)

Here it modifies the verb legas, so the adverb ending -e is needed.

Because the sentence does not need to say what people are reading. Esperanto, like English, can leave the object unstated when it is unimportant or understood from context.

So oni legas trankvile simply means that reading happens quietly. It does not have to specify books, newspapers, or anything else.

Because this sentence talks about location, not movement.

• en la biblioteko = in the library
• en la bibliotekon = into the library

In Esperanto, the accusative -n can show direction toward a place. Since the sentence means that the reading happens inside the library, not that someone is going into it, biblioteko stays without -n.

Esperanto has only one article: la, the definite article. It does not have an indefinite article like English a or an.

So:

• la biblioteko = the library
• biblioteko = a library / library, depending on context

In this sentence, la biblioteko suggests a definite or contextually understood library.

Esperanto word order is fairly flexible. Putting En la biblioteko first sets the scene and gives the location a little more emphasis.

Both of these are grammatical:

• En la biblioteko oni legas trankvile.
• Oni legas trankvile en la biblioteko.

The first version feels a bit more like As for the library, people read quietly there or In the library, people read quietly.

No. In Esperanto, oni is a normal and common word. It does not sound especially stiff or formal the way English one sometimes does.

That is why English translations often use more natural wording such as people or you instead of always using one.

Yes. Esperanto allows some flexibility in adverb placement. For example:

• En la biblioteko oni legas trankvile.
• En la biblioteko oni trankvile legas.

Both are possible. The first is probably the most neutral here. Moving trankvile can slightly change what gets emphasized, but the basic meaning stays the same.