Learn Hardcore Czech: Dnes chci vrátit devět knih. - Today I want to return nine books.

Dnes chci vrátit devět knih. – Neutral, focusing on today.
Chci dnes vrátit devět knih. – Still neutral; a very common order.
Dnes chci vrátit devět knih, ne zítra. – Emphasizes today in contrast to another day.

You usually keep the verb and its infinitive together (chci vrátit) and move adverbs like dnes and objects like devět knih around them.

Why is it chci vrátit and not something like chci vracím?

In Czech, when one verb expresses a wish, plan, ability, etc., it is followed by an infinitive:

chci vrátit – I want to return
můžu vrátit – I can return
musím vrátit – I must return

So the pattern is:

conjugated verb + infinitive
chci (I want) + vrátit (to return)

Using two conjugated verbs together (like chci vracím) is ungrammatical in Czech.

What is the difference between vrátit and vracet?

They differ in aspect:

vracet – imperfective (focus on process or repetition)
- Každý týden vracím knihy. – I return books every week.
vrátit – perfective (focus on a single completed action)
- Dnes chci vrátit devět knih. – I want to (successfully, completely) return nine books today.

In your sentence, you are talking about one concrete action that you intend to complete, so the perfective vrátit is appropriate.

Why is it devět knih and not devět knihy or devět kniha?

Because of how Czech handles numbers + nouns.

For 5 and higher, the noun goes into the genitive plural:

1 – jedna kniha (nominative singular)
2 – dvě knihy (nominative plural)
3 – tři knihy
4 – čtyři knihy
5 – pět knih (genitive plural)
9 – devět knih (genitive plural)

So with devět, the correct form is knih (genitive plural), not knihy or kniha.

What case is knih and how is it formed from kniha?

Knih is genitive plural of kniha (a book).

A simplified part of the paradigm:

Nominative singular: kniha – (one) book
Nominative plural: knihy – books
Genitive plural: knih – of books

After numbers 5 and higher, Czech uses the genitive plural, so:

devět knih = nine (of) books

How would the sentence change with a different number, like two or five books?

The number changes, and sometimes the noun ending changes too.

For 2–4, the noun is in nominative plural:

Dnes chci vrátit dvě knihy. – Today I want to return two books.
Dnes chci vrátit tři knihy. – Today I want to return three books.
Dnes chci vrátit čtyři knihy. – Today I want to return four books.

For 5 and higher, the noun is in genitive plural:

Dnes chci vrátit pět knih. – Today I want to return five books.
Dnes chci vrátit devět knih. – Today I want to return nine books.

Are there articles missing? How would I say the nine books instead of just nine books?

Czech has no articles like a/an or the. The bare devět knih can mean:

nine books
the nine books

Context tells you which one is intended.

If you really want to stress those specific nine books, you can add a demonstrative:

Těch devět knih chci dnes vrátit. – I want to return those nine books today.
Dnes chci vrátit těch devět knih. – Today I want to return those nine books.

But in many situations Dnes chci vrátit devět knih is enough.

Is Dnes chci vrátit devět knih polite enough to say to a librarian?

It is grammatically correct and understandable, but it sounds quite direct: you are simply stating your wish.

For a more polite, “I would like to…” tone, Czech usually uses the conditional of chtít:

Chtěl bych vrátit devět knih. – I would like to return nine books. (said by a man)
Chtěla bych vrátit devět knih. – I would like to return nine books. (said by a woman)

You can add dnes if you need it:

Dnes bych chtěl / chtěla vrátit devět knih.

How do I pronounce the sentence, and are there any tricky sounds?

Pronunciation (approximate, with stress always on the first syllable of each word):

Dnes – [dnes]
- Like English d-ness but with a clear d and s, no extra vowel.
chci – [xtsɪ] or [xtsi]
- ch = a voiceless kh sound, like German Bach.
- c = ts (as in cats).
- Roughly: khtsi in one syllable.
vrátit – ['vraːcɪt]
- vrá – stressed, with long á (hold it longer).
- t is always clear, never like English soft d.
devět – ['dɛvjɛt]
- dě is soft: tongue higher, similar to dye-vyet.

Main stress pattern: DNES chci VRÁ-tit DE-vět KNIH (primary stress on each first syllable).

What is the difference between dnes and dneska?

Both mean today.

dnes – slightly more neutral or formal; fine in writing and speech.
dneska – more colloquial and conversational; very common in everyday speech.

You could say:

Dnes chci vrátit devět knih.
Dneska chci vrátit devět knih.

Both are correct; they differ mainly in style, not in meaning.

How would I say Today I will return nine books, instead of I want to return?

You can use the future tense of a perfective verb:

Dnes vrátím devět knih. – Today I will return nine books.

Here:

vrátím is the 1st person singular (I) future form of perfective vrátit.
Perfective verbs form the future with present endings, but the meaning is future.

So:

Dnes chci vrátit devět knih. – Today I want to return nine books.
Dnes vrátím devět knih. – Today I will return nine books. (a plan / promise)

How do I make the sentence negative, for example Today I don’t want to return nine books?

You negate verbs in Czech mostly by adding ne- to the beginning of the verb:

chci → nechci – I want → I don’t want

So:

Dnes nechci vrátit devět knih. – Today I don’t want to return nine books.

If you wanted to negate the action itself (not the wanting), you could say something like:

Dnes chci nevrátit devět knih, ale jen osm.
– Today I want to not return nine books, but only eight.

But in normal usage, you usually negate chci: nechci.