Jag måste lämna tillbaka boken till biblioteket i morgon.

Lämna tillbaka means return in the sense of giving something back.

• lämna by itself usually means leave or hand over
• lämna tillbaka means give back / return

So:

• Jag måste lämna tillbaka boken = I have to return the book

This works much like English phrasal verbs such as give back.

In Swedish, many verb expressions are built with a main verb plus a particle, and tillbaka is the particle here.

So the normal form is:

• lämna tillbaka = return, give back

In an ordinary sentence, they often stay together:

• Jag måste lämna tillbaka boken

But in some sentence patterns, other words can come between the verb and the particle. That is normal in Swedish and something you will see more of later.

After modal verbs in Swedish, you normally use the infinitive without att.

Common modal verbs include:

• måste = must / have to
• kan = can
• vill = want to
• ska = will / going to / shall
• bör = should

So you say:

• Jag måste lämna tillbaka boken
• Jag kan läsa boken
• Jag vill gå hem

Not:

• Jag måste att lämna

This is similar to English, where you say must leave, not must to leave.

Boken means the book.

Swedish often puts the definite article at the end of the noun as a suffix:

• en bok = a book
• boken = the book

Since the sentence is talking about a specific book, Swedish uses the definite form boken.

Biblioteket means the library.

This is the same definite pattern:

• ett bibliotek = a library
• biblioteket = the library

So the ending -et is the definite form for this ett-word.

Because they belong to different grammatical genders in Swedish.

Swedish nouns are mainly divided into two groups:

• en-words
• ett-words

Examples here:

• en bok → boken
• ett bibliotek → biblioteket

So the definite ending depends on whether the noun is an en-word or an ett-word.

Till here means to.

• till biblioteket = to the library

It shows the destination of the action. You are returning the book to the library.

So:

• Jag måste lämna tillbaka boken till biblioteket = I have to return the book to the library

They look similar, but they do different jobs.

• tillbaka = back
• till = to

So:

• lämna tillbaka = give back / return
• till biblioteket = to the library

A helpful way to see it is:

• back tells you the action is a return
• to the library tells you where the book is being returned

Traditionally, i morgon is written as two words and means tomorrow.

You may also see:

• imorgon

Both are common, and modern Swedish accepts both spellings. Many learners are first taught i morgon, but imorgon is also very widely used.

Time expressions often come later in Swedish sentences, especially after the main information.

So this order is very natural:

• Jag måste lämna tillbaka boken till biblioteket i morgon.

But Swedish is flexible, and you can also move the time expression to the front:

• I morgon måste jag lämna tillbaka boken till biblioteket.

That version gives a little more focus to tomorrow.

This is because Swedish follows the V2 rule in main clauses. That means the finite verb usually comes in the second position.

Original order:

• Jag | måste | lämna tillbaka boken till biblioteket i morgon

If you move I morgon to the front, the verb still has to stay in second position:

• I morgon | måste | jag lämna tillbaka boken till biblioteket

So Swedish says:

• I morgon måste jag lämna tillbaka boken till biblioteket

Not:

• I morgon jag måste...

This is one of the most important word-order rules in Swedish.

Not always. In many everyday situations, måste is often closer to have to than to strong English must.

So:

• Jag måste lämna tillbaka boken i morgon

can often sound natural in English as either:

• I must return the book tomorrow.
• I have to return the book tomorrow.

The exact feeling depends on context.

Normally, no. Swedish usually requires the subject to be stated.

So you say:

• Jag måste lämna tillbaka boken

not simply:

• Måste lämna tillbaka boken

In casual speech or notes, people may drop words, but in normal standard sentences, the subject is usually included.

The written sentence is already natural, but in everyday speech a speaker might say something slightly shorter depending on context, for example:

• Jag måste lämna tillbaka boken imorgon.

If it is already obvious where the book belongs, till biblioteket might be left out.

But if you want the full idea clearly, the original sentence is perfectly normal:

• Jag måste lämna tillbaka boken till biblioteket i morgon.

Måste is pronounced roughly like MOS-teh, but with Swedish sounds.

A few useful points:

• å sounds somewhat like the vowel in British more for many learners
• st here is a normal st sound
• the final -e is pronounced, but lightly

So do not pronounce it like English most. The Swedish å is important.

Yes. It is a very common general expression for returning something.

Examples:

• Jag måste lämna tillbaka nyckeln. = I have to return the key.
• Hon lämnade tillbaka pengarna. = She returned the money.
• När ska du lämna tillbaka bilen? = When are you going to return the car?

So it is not limited to books or libraries.