Learn Hardcore Czech: Dnes si chci půjčit jednu krátkou knihu a zítra ji vrátit. - Today I want to borrow one short book and return it tomorrow.

Czech has special clitic words (short unstressed words) like si, se, mi, ti, ho, ji, jsem, by etc. They normally go in the second position in the clause, right after the first stressed element.

In your sentence:

First stressed element: Dnes
Then clitic: si
Then the verb: chci

So: Dnes si chci půjčit… is correct.

Dnes chci si půjčit… sounds wrong in standard Czech, because si is no longer in that second position.

You can also say:

Chci si dnes půjčit knihu. (first stressed word is Chci, so si comes after it)
Já si dnes chci půjčit knihu. (first stressed word is Já, so si comes after it)

But always keep the clitic si in (roughly) that second position in the clause.

Why is it jednu krátkou knihu and not jedna krátká kniha?

Because jednu krátkou knihu is in the accusative case, used for the direct object of the verb.

The base forms are:
- jedna (one – feminine, nominative)
- krátká (short – feminine, nominative)
- kniha (book – feminine, nominative)

In your sentence, book is the object of půjčit:

(půjčit) co? → jednu krátkou knihu (what? one short book)

For a feminine noun ending in -a, the accusative singular is:

kniha → knihu
jedna → jednu
krátká → krátkou

So everything agrees in gender (feminine), number (singular), and case (accusative):

jednu krátkou knihu

How do I know that knihu is feminine, and how do I decline it?

Kniha is a very typical feminine noun ending in -a. Many Czech feminine nouns end in -a in the nominative singular:

žena (woman)
káva (coffee)
kniha (book)

A simplified declension of kniha (singular) looks like this:

Nominative: kniha – ta kniha je krátká (the book is short)
Accusative: knihu – čtu knihu (I read a book)
Genitive: knihy – bez knihy (without a book)
Dative: knize – k knize (to the book)
Locative: v knize (in the book)
Instrumental: s knihou (with a book)

Here we need the accusative because knihu is the direct object: I want to borrow what? → knihu.

Why is it krátkou knihu and not krátká kniha?

Adjectives in Czech must agree with the noun in gender, number, and case.

Nominative feminine singular:
- krátká kniha – The book is short.
Accusative feminine singular:
- krátkou knihu – I’m reading a short book.

In your sentence, knihu is accusative singular feminine, so the adjective krátký must also be feminine singular accusative:

krátký → krátká (fem. nominative)
krátká → krátkou (fem. accusative)

So we get jednu krátkou knihu.

Is the word order jednu krátkou knihu fixed, or can I say krátkou jednu knihu?

The normal, neutral order is numeral → adjective → noun:

jednu krátkou knihu

You could sometimes move things around for emphasis (e.g. in poetry or very expressive speech), but:

krátkou jednu knihu sounds unusual and marked; it suggests a special emphasis on krátkou (short) in a way that is not natural here.

So in normal speech and writing, keep:

jednu krátkou knihu

What does ji refer to in a zítra ji vrátit, and why ji and not jí?

Ji is a short object pronoun meaning her/it, referring back to knihu (the book).

knihu is feminine singular.
The corresponding short accusative pronoun is ji.

So:

Zítra ji vrátit. = to return it (the book) tomorrow.

Czech has both short (clitic) and long forms of pronouns:

Accusative feminine singular:
- short: ji
- long: ji / její (context-dependent; but in object position after a verb you normally use ji)

Jí with a long í is usually dative (to her, for her):

Dám jí knihu. – I will give her a book. (to her)

So:

ji (short, acc.) – I will return it (the book).
jí (dat.) – I will give it to her.

Why is it zítra ji vrátit and not zítra vrátit ji?

Again, this is about clitics in second position. Ji is a short unstressed pronoun, so it tends to appear right after the first stressed word of the clause.

In the second part:

First stressed word: zítra
Then clitic pronoun: ji
Then the infinitive: vrátit

So:

…a zítra ji vrátit. (natural, standard)

Zítra vrátit ji sounds unnatural in standard Czech, because ji is not in that second position anymore.

Why is the second verb vrátit in the infinitive? Could I say a zítra ji vrátím instead?

Czech frequently uses one finite verb (with person and tense) plus other verbs in the infinitive to express a chain of actions with the same subject:

Dnes si chci půjčit… a zítra ji vrátit.
= Today I want to borrow… and (I want to) return it tomorrow.

The subject I and the verb chci (I want) logically apply to both infinitives: půjčit and vrátit.

You can also say:

Dnes si chci půjčit jednu krátkou knihu a zítra ji vrátím.

This version separates the actions more and states the second one as a simple future fact (I will return it tomorrow), not as part of the same want to… construction. Both are correct, but the original emphasizes:

I want to borrow it today and (I intend) to return it tomorrow.

Why is it půjčit and not půjčovat? And vrátit and not vracet?

This is about aspect in Czech verbs:

půjčit – perfective (a single completed act of lending/borrowing)
půjčovat – imperfective (repeated/ongoing, “to be lending/borrowing (regularly)”)
vrátit – perfective (to return something completely, once)
vracet – imperfective (to be returning, used repeatedly, or in progress)

In your sentence, you are talking about single, complete actions:

borrow a book once, return it once.

Therefore you use the perfective forms:

půjčit (to borrow once)
vrátit (to return once)

If you wanted to talk about a habit, you’d use imperfective forms, for example:

Často si půjčuju (půjčuji) krátké knihy a vracím je hned druhý den.
I often borrow short books and return them the very next day.

Could I omit jednu and say Dnes si chci půjčit krátkou knihu?

Yes, that is completely correct:

Dnes si chci půjčit krátkou knihu.

Here jednu simply emphasizes there is one book. Without jednu, it’s still clearly singular because knihu is singular, so the meaning is roughly the same.

You’d typically keep jednu if you want to stress “one (as opposed to more than one)”:

Dnes si chci půjčit jednu krátkou knihu, ne dvě.
Today I want to borrow one short book, not two.

Can I move dnes and zítra to other positions in the sentence?

Yes, time adverbs like dnes (today) and zítra (tomorrow) are quite flexible. For example:

Dnes si chci půjčit jednu krátkou knihu a zítra ji vrátit. (original)
Já si dnes chci půjčit jednu krátkou knihu a zítra ji vrátit. (emphasis on I)
Já si chci dnes půjčit jednu krátkou knihu a zítra ji vrátit.
Já si chci půjčit jednu krátkou knihu dnes a vrátit ji zítra.

All are possible; the differences are mostly in focus and emphasis. Just remember that when you move things, the clitics (si, ji) still want to stay near the second position of their clause.

How do you pronounce chci and půjčit? They look tricky.

Pronunciation tips:

chci
- ch = a voiceless velar fricative, like German ch in Bach
- c = like ts in cats
- So chci ≈ [x-tsi], said quickly together.
půjčit
- půj – the ů/ú/ů + j makes a long oo-y type sound, roughly like pooy said quickly
- č = ch in church
- i = short i like in bit
- t = ordinary t

Very roughly: POO‑y‑chit, with stress on the first syllable: PŮJ‑čit (Czech almost always stresses the first syllable).