Learn Hardcore Czech: V knihovně se nesmí poslouchat hlasitá hudba, tam se musí číst potichu. - In the library one is not allowed to listen to loud music; there one has to read quietly.

knihovna = “library” (feminine noun, nominative singular)
After the preposition v (“in”), Czech usually uses the locative case to express location.
The locative singular of knihovna is knihovně.

So:

v + knihovna (N) → v knihovně (Loc) = in the library

What is the role of “se” in “se nesmí poslouchat” and “se musí číst”?

Here “se” creates an impersonal / reflexive-passive construction. It turns the sentence into a general rule without saying exactly who must not / must do something.

V knihovně se nesmí poslouchat…
Literally: In the library it-must-not be-listened-to…
Meaning: You / people must not listen…
Tam se musí číst potichu.
Literally: There it-must be-read quietly.
Meaning: You / people must read quietly there.

So “se”:

removes the explicit subject (people, one, you),
makes the rule sound general and impersonal,
is very common in written rules, instructions, and signs.

Comparable English ideas: “One must not…”, “It is not allowed to…”, “You must…” (general you).

Why is it “nesmí” and not “nemůže”?

Both come from verbs that can translate as “can” in English, but they’re used differently:

smět = to be allowed to
- nesmí = is not allowed, must not (prohibition)
moci = to be able to
- nemůže = cannot, is not able to (lack of ability / possibility)

In a rule like this, we are talking about what is allowed, not about physical ability:

V knihovně se nesmí poslouchat hlasitá hudba.
= Loud music is not allowed to be listened to in the library / You must not listen to loud music in the library.

If you said:

V knihovně se nemůže poslouchat hlasitá hudba.
this would sound more like It’s not possible to listen to loud music in the library (for some reason, e.g. no speakers), which is not the intended meaning.

Why is it “poslouchat” and not “slyšet” for “listen to music”?

Czech distinguishes between:

poslouchat = to listen (actively, on purpose)
slyšet = to hear (passively, with your ears)

In English you say “listen to music”, which is active, so in Czech you use:

poslouchat hudbu = to listen to music

If you use slyšet:

slyšet hudbu = to hear music (you happen to hear it, it’s audible)

So:

nesmí se poslouchat hlasitá hudba = loud music must not be listened to (you must not play/listen to it)
nesmí se slyšet hlasitá hudba would sound more like “loud music must not be heard” (strange as a rule; it sounds like trying to prevent it even from being audible).

Why is it “hlasitá hudba” and not “hlasitou hudbu” after “poslouchat”?

This is a key point: the sentence is using a reflexive passive construction, where “hlasitá hudba” is the subject, not a direct object.

Structure:

(V knihovně) se nesmí poslouchat hlasitá hudba.
- hlasitá hudba = nominative singular feminine (subject)
- literally: Loud music must not be listened to.

If we had a normal active sentence with a subject “people”:

Lidé nesmí poslouchat hlasitou hudbu.
- lidé (people) = subject, nominative
- hlasitou hudbu = object, accusative
- literally: People must not listen to loud music.

But in the original sentence:

There is no explicit subject (people, one).
The “se + infinitive” structure plus nesmí creates a passive-like rule.
The thing that is not to be listened to (hlasitá hudba) behaves like the subject.

Therefore it stays in nominative:

hlasitá hudba (N) = “loud music” (as subject) and not hlasitou hudbu (Acc).

Can the word order be “V knihovně nesmí se poslouchat hlasitá hudba”? Where does “se” go?

In Czech, “se” is a clitic: it tends to appear very early in the clause, usually in the so‑called “second position” (after the first stressed word or phrase).

In this sentence, natural options are:

V knihovně se nesmí poslouchat hlasitá hudba.
V knihovně nesmí se poslouchat hlasitá hudba. (possible, but less usual)
Se v knihovně nesmí poslouchat hlasitá hudba. (sounds wrong)

Most natural and neutral here is:

V knihovně se nesmí poslouchat hlasitá hudba.

Similarly for the second clause:

Tam se musí číst potichu.
is better than
Tam musí se číst potichu. (understandable, but less natural)

What does “tam” add in “tam se musí číst potichu”? Could we omit it?

“tam” means “there” and refers back to “v knihovně”.

V knihovně se nesmí poslouchat hlasitá hudba, tam se musí číst potichu.
Literally: In the library you must not listen to loud music; there you must read quietly.

It adds:

a sense of contrast or focus: there (as opposed to somewhere else),
a more spoken, narrative feel: first you name the place, then you point back to it with there.

Yes, you could say instead:

V knihovně se nesmí poslouchat hlasitá hudba, (v knihovně) se musí číst potichu.
or simply split it:
V knihovně se nesmí poslouchat hlasitá hudba. Musí se tam číst potichu.

But using “tam” is natural and avoids repeating “v knihovně”.

What is the difference between “potichu” and “tiše”?

Both can mean “quietly”, but there are nuances:

potichu
- adverb (originally from “po tichu”),
- very common in everyday speech,
- often used with speaking, reading, moving:
  - mluvit potichu – to speak quietly
  - číst potichu – to read quietly
tiše
- adverb from the adjective tichý (quiet),
- a bit more neutral or bookish in some contexts,
- also very common:
  - mluvit tiše – to speak softly/quietly
  - chovat se tiše – to behave quietly

In this specific sentence you could say either:

Tam se musí číst potichu.
Tam se musí číst tiše.

Both are correct; “potichu” sounds a bit more colloquial/typical with this phrase.

Why is “číst” in the infinitive after “musí”? Does “musí” change form?

The verb muset (must, have to) works like a modal verb and is followed by an infinitive:

musím číst – I must read
musíš číst – you (sg.) must read
musí číst – he/she/it/they/you (formal) must read

Here we have an impersonal construction with “se”:

(Tam) se musí číst potichu.
- musí = 3rd person singular (or plural; same form in present)
- číst = infinitive
- “it must be read” / “one must read” quietly

So:

musí does not change because there is no personal subject (like I, you, they).
The infinitive “číst” is required: after “musí” you always keep the full infinitive form of the main verb (číst, psát, mluvit, etc.).

Why is there no explicit subject like “lidé” (people) or “vy” (you)?

Czech often leaves the subject implicit when:

The subject is obvious from the verb ending, or
The sentence is a general rule using an impersonal construction.

Here we have case (2):

V knihovně se nesmí poslouchat hlasitá hudba, tam se musí číst potichu.

The “se + verb” structure is a typical way to express:

general rules
instructions
impersonal statements

Equivalent English translations would use:

general “you”: You must not listen… you must read quietly.
“one”: One must not listen…
or passive: Loud music must not be listened to; one must read quietly there.

Adding “lidé” or “vy” would make the sentence sound less like a neutral rule and more like addressing a specific group:

Lidé v knihovně nesmí poslouchat hlasitou hudbu.
Vy v knihovně nesmíte poslouchat hlasitou hudbu.

These are correct but stylistically different.

Could we also say “V knihovně je zakázáno poslouchat hlasitou hudbu”? How does that compare?

Yes, this is a very natural alternative:

V knihovně je zakázáno poslouchat hlasitou hudbu. = It is forbidden (prohibited) to listen to loud music in the library.

Comparison:

Original:
- V knihovně se nesmí poslouchat hlasitá hudba.
- Uses nesmí se + infinitive (impersonal reflexive).
- Slightly more neutral/colloquial in everyday speech.
Alternative:
- V knihovně je zakázáno poslouchat hlasitou hudbu.
- Uses je zakázáno = is forbidden (passive predicate).
- Feels a bit more formal / official, typical for written notices and regulations.

Both clearly express a prohibition; the difference is mostly one of style and tone, not meaning.