quantus

quantus, quanta, quantum is a regular first-and-second declension adjective doing two jobs. As an interrogative it asks "how great?", and inside an indirect question it pulls the SUBJUNCTIVE after it: vide quantum flumen sit. As a relative it answers tantus, giving the correlative pair tantus ... quantus, "as great as".

Latin runs a tidy grid of these pairs, and learning the grid is quicker than learning the words separately. tantus/quantus measures size or amount, tot/quot counts things, talis/qualis describes quality, tam/quam grades a degree. The neuter quantum works as an adverb, "as much as", and the genitive quanti is a genitive of price: quanti constat?, "what does it cost?".