Learn Hardcore Czech: V nemocnici se musí mluvit potichu a nesmí se mluvit nahlas, protože tam lidé často spí. - In the hospital one has to speak quietly and must not speak loudly, because people there often sleep.

V nemocnici se musí mluvit potichu.
≈ In hospital, one must speak quietly / People must speak quietly / It is necessary to speak quietly.
V nemocnici se nesmí mluvit nahlas.
≈ In hospital, one must not speak loudly / It is forbidden to speak loudly.

This se is often called “reflexive passive” or impersonal se. It lets you talk about what people in general must / must not do, without saying who exactly.

Compare:

Lidé musí mluvit potichu. – People must speak quietly.
V nemocnici se musí mluvit potichu. – (In general) You must speak quietly in a hospital.

So here se helps express a general rule, not a reflexive action like washing oneself.

Why is there no subject like lidé (people) in the first part of the sentence?

Because Czech often uses impersonal constructions with se for general rules or habits:

V nemocnici se musí mluvit potichu.
Literally: In the hospital, it must be spoken quietly.

This is similar to English “You must speak quietly in hospital” or “People must speak quietly in hospital”. English usually needs a subject (you, people, one), but Czech can avoid it with se.

If you add a subject, it is still correct, but it sounds less like a neutral rule and more like a direct statement about that subject:

V nemocnici lidé musí mluvit potichu. – People in hospitals must speak quietly.
V nemocnici se musí mluvit potichu. – In hospitals, it is required to speak quietly. (more rule-like)

Why is mluvit in the infinitive and not conjugated as mluví?

After musí / nesmí in this impersonal se structure, Czech uses the infinitive of the main verb:

musí se mluvit – it must be spoken / one must speak
nesmí se mluvit – it must not be spoken / one must not speak

If you conjugate mluvit, you get a normal personal sentence:

Lidé mluví potichu. – People speak quietly.
Lidé musí mluvit potichu. – People must speak quietly.

But when you say:

V nemocnici se musí mluví ❌ – this is ungrammatical.
V nemocnici se musí mluvit ✅ – correct, because musí is conjugated and the main verb mluvit stays in the infinitive.

What is the exact difference between musí and nesmí here?

musí = must, have to
V nemocnici se musí mluvit potichu.
→ It is obligatory to speak quietly.
nesmí = must not, may not, is not allowed
V nemocnici se nesmí mluvit nahlas.
→ It is forbidden to speak loudly.

Be careful not to confuse:

nesmí – must not (prohibited)
nemusí – don’t have to (no obligation, but allowed)

For example:

V nemocnici se nesmí mluvit nahlas. – Speaking loudly is forbidden.
V nemocnici se nemusí mluvit potichu. – You don’t have to speak quietly (but you may if you want).

In your sentence, nesmí clearly expresses a rule/ban.

Why is it v nemocnici and not v nemocnice or v nemocnicií?

Nemocnice (hospital) is a feminine noun that has the same form in Nominative and Locative in the singular:

Nominative: nemocnice – (Ta) nemocnice je velká. – The hospital is big.
Locative: v nemocnici – Jsem v nemocnici. – I am in the hospital.

The ending -i after the preposition v shows that this is Locative singular (where?).

So:

v nemocnici ✅ (in the hospital – Locative)
v nemocnice ❌
v nemocnicií ❌ (this ending doesn’t exist)

What part of speech are potichu and nahlas? Why not adjectives like tichý or hlasitý?

Potichu and nahlas are adverbs, and with verbs like mluvit (to speak), Czech normally uses adverbs, not adjectives.

mluvit potichu – to speak quietly
mluvit nahlas – to speak loudly

Adjectives would describe a noun:

tichý pokoj – a quiet room
hlasitý člověk – a loud person

So:

mluvit tichý ❌ – wrong, mixing verb + adjective
mluvit potichu ✅ – verb + adverb

Can I say mluvit tiše instead of mluvit potichu, and mluvit hlasitě instead of mluvit nahlas?

Yes, these pairs are very close in meaning:

mluvit potichu ≈ mluvit tiše – to speak quietly
mluvit nahlas ≈ mluvit hlasitě – to speak loudly

Nuance and frequency:

potichu and nahlas sound very natural and are extremely common in everyday speech.
tiše and hlasitě can sound a bit more neutral / bookish, but are still perfectly correct.

In your sentence, you could say:

V nemocnici se musí mluvit tiše a nesmí se mluvit hlasitě… ✅
It’s correct, just slightly different in style. The original with potichu / nahlas sounds very natural and colloquial-standard.

What is the role of tam in protože tam lidé často spí? Could I move it?

Tam means there and refers back to v nemocnici:

protože tam lidé často spí
→ because people often sleep there (i.e., in the hospital).

You can move tam in the sentence; Czech word order is flexible:

protože tam lidé často spí
protože lidé tam často spí
protože lidé často spí tam

All are grammatically correct. The most neutral-sounding here is probably the original protože tam lidé často spí, putting tam early to connect clearly to the previous v nemocnici.

Can I say V nemocnici musíš mluvit potichu a nesmíš mluvit nahlas instead? What changes?

Yes, that is correct, but the meaning is more personal and direct:

V nemocnici se musí mluvit potichu…
→ General rule: In hospitals, one must speak quietly.
V nemocnici musíš mluvit potichu…
→ Direct rule to you (ty): You must speak quietly in the hospital.

So:

se musí mluvit → impersonal, general rule, like a sign or regulation
musíš mluvit → you (singular, informal) personally must do it

Both are good; you choose depending on whether you are speaking generally or directly to someone.

Why is there a comma before protože in this sentence?

In Czech, subordinate clauses introduced by conjunctions like protože (because), když (when), že (that), etc., are normally separated by a comma.

Your sentence has:

Main clause: V nemocnici se musí mluvit potichu a nesmí se mluvit nahlas
Subordinate clause: protože tam lidé často spí

So a comma before protože is required:

V nemocnici se musí mluvit potichu a nesmí se mluvit nahlas, protože tam lidé často spí. ✅

Why is it často spí and not spí často? Does the position of často change the meaning?

Both lidé často spí and lidé spí často are grammatically correct.

často spí (adverb before verb) is probably more common and neutral here.
spí často can put a bit more emphasis on how often they sleep, or just sound slightly more marked in this context.

The basic meaning doesn’t change; both mean “people often sleep”. Word order in Czech is quite flexible and is often used to express emphasis or rhythm rather than strict grammatical rules.

Could I use říkat instead of mluvit, like musí se říkat potichu?

You can say říkat in some contexts, but here mluvit is more natural.

mluvit – to speak (general act of speaking)
říkat – to say (specific statements, words, messages)

The rule in a hospital is about speaking in general, not just about specific things you say, so mluvit fits better:

V nemocnici se musí mluvit potichu. ✅
You must speak quietly.

If you said:

V nemocnici se musí říkat potichu.
It sounds more like Things must be said quietly – grammatically possible but stylistically odd for a general rule.