Learn Hardcore Czech: Knihy musím vrátit včas, protože to je hlavní pravidlo knihovny. - I have to return the books on time because that is the main rule of the library.

Knihy musím vrátit včas. – puts focus on knihy (“The books I must return on time”), as if you were contrasting with something else (not magazines, not DVDs, but the books).
Musím vrátit knihy včas. – more neutral, “I have to return the books on time,” like normal English word order.

So starting with knihy is a way to emphasize what must be returned.

What case is knihy, and why does it have that form?

Knihy is in the accusative plural, because it is the direct object of the verb vrátit (to return something).

The noun kniha (a book) declines like this in the singular/plural:

Nominative sg: kniha – “(the) book” (subject)
Accusative sg: knihu – “(the) book” (object)
Nominative pl: knihy – “(the) books” (subject)
Accusative pl: knihy – “(the) books” (object)

In the plural, nominative and accusative are the same (knihy). Here, because it is “I must return the books,” we know it’s accusative from the role in the sentence, not from its form.

Why is there no word for “I” in musím vrátit?

Czech usually omits subject pronouns when the verb ending already shows the person:

(Já) musím = I must
(Ty) musíš = you must
(On) musí = he must

The -ím ending in musím clearly indicates 1st person singular (“I”), so já is unnecessary unless you want to emphasize it:

Já musím vrátit knihy včas. – I (as opposed to someone else) have to return the books on time.

Why is vrátit in the infinitive after musím?

Modal verbs like muset (must), moci (can), chtít (want) are normally followed by an infinitive, just like in English:

musím vrátit – I must return
můžu přijít – I can come
chci číst – I want to read

So musím vrátit is “I must (to) return,” where vrátit is the infinitive.

Why is it vrátit (perfective) and not vracet (imperfective)?

Czech has aspect: perfective vs imperfective.

vrátit – perfective: one completed act of returning (to return once, finish the action)
vracet – imperfective: an ongoing, repeated, or habitual action (to be returning, to return regularly)

In this sentence, the rule is about completing the act by the deadline (returning the books in time once), so perfective vrátit fits:

Knihy musím vrátit včas. – I must (successfully) return the books on time (by the due date).

If you said:

Knihy musím vracet včas.

it would sound more like a repeated obligation: “I have to (always/regularly) return books on time.”

What exactly does včas mean? How is it different from načas or časem?

včas means “on time, before it’s too late / before the deadline.” It’s used for doing something early enough:

Přišel jsem včas. – I arrived on time.
Musím to stihnout včas. – I must manage it in time.

Related words:

načas – very close to “on time” too, often “at the right time / punctually”. In many contexts it overlaps with včas, but včas strongly suggests “not too late” (before a negative consequence).
časem – “in time, eventually, over time,” with a slow-progress meaning:
- Časem si zvykneš. – You’ll get used to it in time / eventually.

So in the library context, včas is the natural choice: returning before the due date expires.

Can I say Musím vrátit knihy včas instead? Is there any difference in meaning?

Yes, Musím vrátit knihy včas. is fully correct.

Nuance:

Knihy musím vrátit včas. – topicalizes/emphasizes knihy (the books).
Musím vrátit knihy včas. – more neutral word order, similar to English.

Meaning-wise they both express the same obligation.

Why is it protože to je and not just protože je?

In the clause protože to je hlavní pravidlo knihovny, to functions as the subject pronoun (“this / that / it”), referring to the whole previous idea (“having to return the books on time”).

If you said only:

protože je hlavní pravidlo knihovny

it sounds incomplete and unnatural, because there is no clear subject. You need something like:

protože to je hlavní pravidlo knihovny – because this is the main rule of the library
or
protože hlavní pravidlo knihovny je vrátit knihy včas – because the main rule of the library is to return the books on time.

In the given structure, to is required to anchor the sentence to the earlier statement.

What’s the difference between protože to je hlavní pravidlo and protože je to hlavní pravidlo?

Both are grammatically correct, and both can appear here:

protože to je hlavní pravidlo knihovny
protože je to hlavní pravidlo knihovny

The difference is in word order and emphasis:

to je (TO je hlavní pravidlo…) – stresses “this”: “because this is the main rule of the library.” You are presenting “this” as your main justification.
je to (…je TO hlavní pravidlo) – stresses the predicate “the main rule”, more like “because it is the main rule of the library (and not just a minor suggestion).”

In everyday speech, both versions are very common, and the difference is subtle and mostly about intonation and focus.

What case is knihovny, and how does it mean “of the library”?

Knihovny here is genitive singular of knihovna (library).

Nominative sg: knihovna – the library (subject)
Genitive sg: knihovny – of the library

In Czech, the genitive is regularly used to show possession or association between two nouns:

pravidlo knihovny – the rule of the library
dveře auta – the door of the car
okno domu – the window of the house

So hlavní pravidlo knihovny literally is “the main rule of the library.”

Why doesn’t hlavní change its form here? Does it always look like hlavní?

Hlavní is an adjective that belongs to a group often called “soft adjectives” (ending in -í in the base form). These adjectives often look the same in many forms, especially in the nominative and accusative.

Here:

pravidlo is neuter singular nominative.
hlavní agrees with pravidlo (neuter singular nominative), but its form doesn’t visibly change – it stays hlavní.

Examples with different genders/numbers:

hlavní pravidlo – the main rule (neuter sg)
hlavní knihovna – the main library (feminine sg)
hlavní město – the capital city (neuter sg)
hlavní knihy – the main books (plural)

They all use hlavní. It does decline in the full paradigm, but for a learner, it often feels “invariable” because the form hlavní appears in many positions.

Could I say pravidlo v knihovně instead of pravidlo knihovny? What’s the difference?

You can say both, but they’re not identical:

pravidlo knihovny – literally “the rule of the library,” a rule that belongs to or is set by the library as an institution. This is exactly what you want for library regulations.
pravidlo v knihovně – “a rule in the library,” a rule that operates in that place. It describes location more than ownership.

Sometimes both can be used, but with different nuance. Compare:

Hlavní pravidlo knihovny je ticho. – The library’s main rule is silence. (institutional rule)
Hlavní pravidlo v knihovně je ticho. – The main rule in the library is silence. (in that environment; stylistically also fine)

In your sentence, pravidlo knihovny is the more typical way to talk about official library rules.

What tense is musím vrátit? It looks like present, but it refers to the future. How does that work?

Formally, musím is present tense of muset (I must).

However, in Czech:

A present-tense modal verb (musím, můžu, chci) + an infinitive often refers to a future obligation/action:
- Zítra musím vrátit knihy. – I have to return the books tomorrow.
- Musím to udělat do pátku. – I must do it by Friday.

In your sentence, musím vrátit describes an obligation that applies now, typically about something you will do by some future time (before the due date). The combination present-tense modal + infinitive is the normal way to express this.

If I replace knihy with a pronoun, where does je go? Can I say Knihy je musím vrátit včas?

If you replace knihy with a pronoun, you usually say:

Musím je vrátit včas. – I must return them on time.

The pronoun je (them) is a clitic; in neutral sentences it tends to appear early, usually in the “second position”:

Dnes je musím vrátit včas. – Today I must return them on time.
Zítra je musím vrátit.

If you keep knihy, you normally don’t add je, because that would be redundant:

Knihy musím vrátit včas. – correct
Knihy je musím vrátit včas. – sounds wrong in this structure

You can have both knihy and je only in special, strongly emphatic constructions, usually with a pause/comma and a repeated subject, e.g.:

Ty knihy, já je musím vrátit včas. – Those books, I have to return them on time.

But that is a different, more emphatic sentence type. For the normal sentence, you either say Knihy musím vrátit včas. or Musím je vrátit včas., not both together.