Mi prunteprenas tiun libron.

Prunteprenas comes from pruntepreni:

• prunte = as a loan
• preni = to take

So literally it means to take as a loan, which is how Esperanto expresses to borrow.

The full form here is prunteprenas:

• pruntepren- = the verb stem
• -as = present tense

So Mi prunteprenas tiun libron means I borrow / I am borrowing that book.

Esperanto very often builds vocabulary by combining simpler elements into one compound word. That is what happens here:

• prunte
  • preni → pruntepreni

This is normal and very common in Esperanto. Instead of memorizing a completely unrelated word for borrow, you can often understand the meaning from the parts.

This is one of the language’s most learner-friendly features.

The ending -as marks the present tense.

So prunteprenas can mean:

• I borrow
• I am borrowing

Esperanto does not force the same tense/aspect distinction that English does here. The exact English translation depends on context.

Because tiun libron is the direct object of the sentence: it is the thing being borrowed.

In Esperanto, the direct object takes -n. Also, words that describe or point to the noun agree with it, so both words get -n:

• tiu libro = that book
• tiun libron = that book as a direct object

So in this sentence:

• Mi = subject
• prunteprenas = verb
• tiun libron = direct object

That is why both words have the accusative ending.

Because tiu libro would be the basic nominative form, used when the phrase is a subject or is not a direct object.

Here, the book is the thing being borrowed, so it must be in the accusative:

• Tiu libro estas nova. = That book is new.
• Mi prunteprenas tiun libron. = I borrow that book.

So the sentence needs tiun libron, not tiu libro.

Because tiu already makes the noun definite. It means that or that one, so adding la would normally be unnecessary.

Compare:

• libro = a book / book
• la libro = the book
• tiu libro = that book

So tiun libron already has a definite meaning by itself.

Because mi is the subject of the sentence, not the direct object.

In Esperanto:

• mi = I
• min = me

So:

• Mi prunteprenas tiun libron. = I borrow that book.
• Li vidas min. = He sees me.

Only the direct object gets -n, so mi stays mi here.

Yes. Esperanto word order is fairly flexible because the -n ending shows what the direct object is.

The most neutral order is:

• Mi prunteprenas tiun libron.

But you could also say:

• Tiun libron mi prunteprenas.

That would put more emphasis on that book.

Even though the word order changes, tiun libron is still the object because of the -n ending.

By itself, tiu usually means that.

If you want to be clearly this, Esperanto often uses:

• ĉi tiu = this

So:

• tiu libro = that book
• ĉi tiu libro = this book

In your sentence, tiun libron most naturally means that book.

The opposite is pruntedoni, which means to lend.

Compare:

• Mi prunteprenas tiun libron. = I borrow that book.
• Mi pruntedonas tiun libron. = I lend that book.

This is another nice example of Esperanto word-building:

• preni = take
• doni = give
• pruntepreni = borrow
• pruntedoni = lend