First, it is not the definite article but a demonstrative adjective (or demonstrative pronoun according to Czech nomenclature), which overlaps with that/those. Definiteness can also be expressed by the word order. Yeah, insane. This demonstrative adjective reflects the gender, number, and case. In the nominative, the forms are TEN, TA, TO (ten kluk, ten dům, ta holka, to jablko) in the singular and TI, TY, TA (ti kluci, ty domy, ty holky, ta jablka) in the plural. See section 5.4.1 in the Naughton book that you already mentioned for the other cases. Good luck.
Ale vždyť v odkazu který jste zde dal je napsáno koni. Tak to jsem z toho mimo :(