In bibliotheca hic codex lente legendus est, quia scriptura eius difficilis est.

Because in takes the ablative when it means in or on in the sense of location.

• in bibliotheca = in the library
• If in showed motion into something, Latin would use the accusative instead:
  • in bibliothecam = into the library

So here the sentence is talking about where the codex is to be read, not movement toward the library.

Hic is a demonstrative adjective meaning this. It has to agree with the noun it modifies in gender, number, and case.

• codex is masculine singular nominative
• so the form must be hic

That is why you get:

• hic codex = this codex / this book

Not:

• haec codex, because haec is feminine nominative singular (or neuter plural)
• hoc codex, because hoc is neuter nominative singular

Because codex is the subject of legendus est.

In English, we often think of read as taking a direct object, so a learner may expect the book to be accusative. But this is not a normal active verb form. It is a passive periphrastic:

• codex legendus est = the codex must be read / the codex is to be read

In that construction, the thing that has to be read becomes the subject, so it is nominative.

It is a gerundive + form of esse, usually called the passive periphrastic.

• legendus comes from legere = to read
• legendus means something like to be read or needing to be read
• est means is

Together:

• legendus est = must be read, should be read, or more literally is to be read

This construction often expresses necessity, obligation, or appropriateness.

Because it agrees with codex, which is:

• masculine
• singular
• nominative

The gerundive behaves like an adjective, so it must match the noun it goes with.

If the noun were feminine singular, you would expect legenda.
If it were neuter singular, you would expect legendum.

So:

• hic codex legendus est = this codex must be read

Because lente is an adverb, and it modifies the verbal idea legendus est.

• lentus = slow as an adjective
• lente = slowly as an adverb

So:

• lente legendus est = must be read slowly

An adjective would describe a noun; an adverb describes how the action is done.

Latin does not have to state the person responsible. The sentence can leave the obligation general or understood from context.

So hic codex legendus est can mean:

• this codex must be read
• this codex should be read

If Latin wanted to say who has the duty, it could add a dative of agent:

• nobis hic codex legendus est = this codex must be read by us / we must read this codex

Without such a dative, the sentence simply states the necessity.

This is a reason clause introduced by quia, meaning because.

• quia = because
• scriptura eius = its writing or its script
• difficilis est = is difficult

So the clause gives the reason for reading slowly: the writing is difficult.

A learner should also notice that difficilis agrees with scriptura:

• scriptura is feminine singular nominative
• difficilis is a third-declension adjective whose masculine/feminine nominative singular is difficilis

Because suus is a reflexive possessive and normally refers to the subject of its own clause.

In the clause:

• scriptura eius difficilis est

the subject is scriptura, not codex.

If you used sua, it would naturally refer back to scriptura itself, not to codex. But the meaning here is the codex’s writing, so Latin uses eius = his/her/its.

So:

• scriptura eius = its writing
• not scriptura sua, which would suggest a reflexive relationship inside that clause

No. Although the words are related historically, scriptura in this sentence means something like:

• writing
• handwriting
• script
• style of writing

So scriptura eius difficilis est means the writing or script of the codex is hard to read, not that some religious scripture is difficult.

Because quia commonly introduces a straightforward factual reason, and in that use Latin normally takes the indicative.

Here the speaker presents the difficulty of the writing as a real fact:

• quia scriptura eius difficilis est = because its writing is difficult

The subjunctive is more likely in other kinds of subordinate clauses, but a simple quia clause giving an actual reason usually uses the indicative.

Yes, but not in a way that changes the basic grammar. Latin word order is more flexible than English word order, so placement often shows emphasis or style rather than strict syntactic necessity.

Possible effects here:

• In bibliotheca comes first to set the scene: in the library
• hic codex then points out the item under discussion: this codex
• lente legendus est places the obligation near the center of the sentence
• quia... comes afterward as the explanation

You could rearrange many parts and still keep the same core meaning, though the emphasis would shift.

Because Latin has no articles.

So:

• codex can mean a codex, the codex, or simply codex/book, depending on context
• bibliotheca can mean a library or the library

English requires articles, but Latin usually leaves that distinction to context. Here English would probably use the or this naturally, but Latin does not need a separate word for it.