"The book is in the market."

Translation: Liber est in foro.

[Ghali623998]

What's the difference between "liber" and "libro" ?

[Moopish]

The grammatical case.

Liber could be the nominative singular (subject of the sentence) or vocative singular (direct reference, 'Hey book!').

Libro could be the dative singular (indirect object, 'to/for the book') or ablative singular ('by/with/from the book', used with lots of prepositions).

[FadeToGreyX]

Why is "librum" incorrect?

[ekoi1995]

Case	Singular	Plural
Nominative	liber	librī
Genitive	librī	librōrum
Dative	librō	librīs
Accusative	librum	librōs
Ablative	librō	librīs
Vocative	liber	librī

Case	Singular	Plural
Nominative	forum	fora
Genitive	forī	forōrum
Dative	forō	forīs
Accusative	forum	fora
Ablative	forō	forīs
Vocative	forum	fora

[Eugenio295078]

Liber in foro est: is it better?

[jsclingman]

Would forum be the accusative version of foro?

[Moopish]

Yes.

forum can be the nominative, accusative, and vocative singular, while foro can be the dative or ablative singular.