Qualis liber tibi hodie placet?

Qualis means what sort of or what kind of. It is asking about the type or character of the book, not just its identity.

So Qualis liber tibi hodie placet? is more like:

• What kind of book do you like today?

If you wanted which book?, Latin would more naturally use qui rather than qualis.

Liber is the subject of placet.

That may feel strange to an English speaker, because in English we say:

• You like the book

But Latin uses placere differently:

• The book pleases you

So the thing that is liked, liber, becomes the grammatical subject, and therefore it is in the nominative singular.

Because placere takes the person who experiences the liking in the dative case, not the nominative.

So:

• tibi = to you / for you
• tu = you as the subject

Latin is literally saying:

• What kind of book is pleasing to you today?

That is why tibi is correct.

Placet means it pleases or is pleasing.

It comes from the verb placeo, placere, which means to please.

In this sentence, placet is:

• present tense
• third person singular
• active voice
• indicative mood

It is third person singular because the subject, liber, is singular.

Because the subject is liber, which is singular.

Latin verbs agree with their subject in number, so:

• liber placet = the book pleases
• libri placent = the books please

If the sentence were about multiple books, you would need placent instead of placet.

Hodie is an adverb meaning today.

It tells you when the liking applies. So the sentence is asking what kind of book appeals to you today, perhaps implying that your preference might vary from day to day.

Because it is an adverb, it does not change form.

Not in the same way it is in English. Latin word order is relatively flexible because the endings show the grammatical roles.

So these would all be understandable Latin with roughly the same basic meaning:

• Qualis liber tibi hodie placet?
• Hodie qualis liber tibi placet?
• Tibi qualis liber hodie placet?

That said, the given order is very natural:

• qualis liber puts the question word first
• tibi comes early because it is important to the meaning
• hodie sits naturally before the verb

Here it means book.

Latin has two different words spelled liber, but they are different words:

• liber = book
• līber = free

In normal printed Latin for learners, the long vowel may be marked with a macron:

• liber = book
• līber = free

In your sentence, the meaning and grammar clearly show that liber means book.

Latin does not have articles like English a, an, or the.

So liber can mean:

• a book
• the book

Which one is meant depends on context.

In this sentence, English usually translates it naturally as what kind of book, but Latin itself does not include a separate word for a or the.

It mainly asks what kind of book.

That is one of the most important distinctions for an English-speaking learner:

• qualis liber? = what kind of book?
• qui liber? = which book?

So qualis is about quality, type, or character, not primarily about choosing one specific item from a set.

A natural answer would usually keep the same placere pattern, with the person in the dative and the thing liked as the subject.

For example:

• Mihi liber longus placet. = I like a long book.
• Mihi liber de historia placet. = I like a book about history.
• Mihi hodie liber levis placet. = Today I like a light/easy book.
• Mihi fabulae magis placent. = I like stories more.

Notice again that Latin says literally:

• A long book is pleasing to me
• Stories are more pleasing to me

rather than using the English-style structure I like ...

Qualis is nominative masculine singular, because it agrees with liber.

Since qualis is describing liber, it must match it in:

• gender: masculine
• number: singular
• case: nominative

So:

• qualis liber = what kind of book

If the noun changed, qualis would change too where necessary. For example:

• quale consilium = what kind of plan
• quales libri = what kind of books