Quidam in bibliotheca tacent, alii de fabula disputant.

Quidam is an indefinite pronoun/adjective meaning some, certain, or some people.

In this sentence it is being used substantively, which means it stands by itself without an expressed noun. So quidam means something like some people or certain people.

A useful detail: quidam can be either masculine singular nominative or masculine plural nominative, so the form by itself is ambiguous. We know it is plural here because the verb with it is tacent, which is third person plural: some people are silent, not a certain person is silent.

Alii means others. It is the nominative masculine plural form of alius.

Like quidam, it is also being used without an expressed noun, so it means other people or simply others.

The sentence sets up a contrast:

• quidam = some people
• alii = other people / others

So this is a very common Latin pattern of dividing a group into two parts: some ... others ...

Because in takes the ablative when it shows location.

So:

• in bibliotheca = in the library
• in
  • ablative = being in a place

Compare this with motion into a place:

• in bibliothecam = into the library

That contrast is very important in Latin:

• ablative after in = where?
• accusative after in = into where?

Here the people are already located in the library, so bibliotheca is ablative.

Because the preposition de normally takes the ablative.

So:

• de fabula = about the story / about the play / about the fable

The exact English word depends on context, because fabula can mean several related things, such as story, tale, fable, or play.

The main grammar point is simple: after de, Latin uses the ablative.

Both are third person plural present active indicative verbs.

• tacent comes from taceo, tacere = to be silent, to keep quiet
• disputant comes from disputo, disputare = to discuss, debate, or sometimes argue

Because both verbs are plural, they match the plural subjects quidam and alii.

So the structure is:

• quidam ... tacent = some people are silent
• alii ... disputant = others are discussing

Yes, you can think of an understood noun such as people, students, men, or whatever group the context suggests.

Latin often uses words like quidam and alii by themselves when the noun is obvious from context.

Since they are masculine plural, they can refer to:

• a group of men, or
• a mixed group of people

If the group were clearly feminine, Latin would use feminine forms instead:

• quaedam ... aliae ...

Classical Latin does not have articles like English the or a/an.

So:

• in bibliotheca can mean in the library or in a library
• de fabula can mean about the story or about a story

English has to choose an article, but Latin usually leaves that to context.

This is why Latin sentences can feel a little more compact than English ones.

Yes. Latin word order is much more flexible than English word order because the endings show the grammatical relationships.

This sentence puts the two groups first and then gives their actions:

• Quidam ... tacent
• alii ... disputant

That makes the contrast very clear.

You could rearrange the words and still keep the basic meaning, for example:

• Alii de fabula disputant, quidam in bibliotheca tacent.

But the emphasis would shift a little. Latin word order is often used for style, focus, and contrast rather than for basic grammatical clarity.