"Matěj má mnoho svetrů."

Translation: Matěj has many sweaters.

[lkd404479]

Why is the genitive case used here instead of accusative?

[nueby]

Svetrů must be in the genitive because it is used with a generic expression of quantity. Mnoho, málo, několik, kolik, and numbers from 5 upward are all used with the genitive. The phrase "mnoho svetrů" as a whole is in the accusative, as that is what the verb "má" requires. We have something like case nesting going on here.

[ion1122]

"mnoho" is not really an adjective like the English "many". It is an adverb. The literal meaning of the Czech is something like "He has muchly of sweaters." Or, if you prefer, translate "mnoho" as "lots of", so you can see the genitive marker "of".