В библиотеке нельзя нарушать тишину, поэтому я говорю шёпотом.

In the library you must not disturb the silence, so I speak in a whisper.

Breakdown of В библиотеке нельзя нарушать тишину, поэтому я говорю шёпотом.

• я: I
• говорить: to speak
• в: in
• библиотека: the library
• тишина: the silence
• шёпотом: in a whisper
• нельзя: must not / not allowed
• поэтому: so / therefore
• нарушать: to disturb / to break

Questions & Answers about В библиотеке нельзя нарушать тишину, поэтому я говорю шёпотом.

Why is it В библиотеке and not в библиотека or в библиотеку?

Because в changes meaning depending on the case:

• в + Prepositional (в библиотеке) = in the library (location, “where?”).
• в + Accusative (в библиотеку) = to the library (direction, “where to?”). Here we mean a location, so в библиотеке is correct.

What does нельзя mean grammatically, and why is there no subject like мы/ты/я?

Нельзя is an impersonal word meaning it’s not allowed / one must not / it’s forbidden.
It often forms an impersonal construction: нельзя + infinitive.
Because it’s impersonal, you don’t need an explicit subject; it’s understood as “people in general.”

Why is it нельзя нарушать (infinitive) and not something like нельзя нарушаю?

After нельзя, Russian normally uses the infinitive: нельзя + infinitive = it’s not allowed to do X.
A conjugated verb like нарушаю would mean I break (it) and doesn’t fit the “forbidden to…” pattern.

What does нарушать тишину literally mean, and is it a fixed phrase?

Literally it’s to violate/break silence. In natural English you’d usually say to make noise / to disturb the silence / to be noisy.
It’s a common Russian collocation: нарушать тишину = disturb the quiet.

Why is it нарушать (imperfective aspect) and not нарушить (perfective)?

With rules/prohibitions, Russian typically uses the imperfective to talk about an action in general:

• нельзя нарушать тишину = you must not (ever) disturb the silence / don’t be noisy (as a rule).
  Perfective нельзя нарушить is possible but sounds more like “not allowed to commit that single violation,” less natural for a general rule.

Why is there a comma before поэтому?

Because поэтому here introduces a second clause that gives the result/conclusion:
..., поэтому я говорю... = ..., therefore I speak...
Two clauses → comma is standard.

Is поэтому the same as потому что?

No—these are opposites in cause/effect direction:

• потому что = because (gives the reason): Я говорю шёпотом, потому что в библиотеке нельзя...
• поэтому = therefore/so (gives the result): В библиотеке нельзя..., поэтому я говорю...

Why does it say я говорю шёпотом and not я шепчу?

Both can work, but they’re slightly different:

• говорю шёпотом = I speak in a whisper / I speak quietly (whispering) (describes manner).
• шепчу = I whisper (focuses on the act of whispering itself).
  The sentence chooses a neutral “speaking” verb + manner.

What case is шёпотом, and why is it in that form?

Шёпотом is Instrumental case of шёпот (a whisper).
Instrumental is often used to express manner/means:

• говорить шёпотом = to speak in a whisper Similar: говорить тихим голосом (in a quiet voice).

Why is тишину in the accusative?

Because нарушать is a transitive verb that takes a direct object.
Direct objects of non-negated transitive verbs are typically Accusative:

• нарушать (что?) тишину

Can I drop я and just say ... поэтому говорю шёпотом?

Yes, often. Russian frequently omits subject pronouns when the verb ending already shows the person:

• ... поэтому говорю шёпотом is natural and means the same.
  Keeping я adds a bit of emphasis: “so I (personally) speak in a whisper.”

What’s the deal with ё in шёпотом—can it be written as шепотом?

In many texts ё is often written as е, so you may see шепотом.
But it’s pronounced shyópотом (with yo), and using ё helps avoid ambiguity and shows correct stress.