"Those dogs do not eat cheese."
Translation:Quei cani non mangiano formaggio.
Is it the case that 'quei' is used in adjective form and 'quelli' as a pronoun and that's why you cannot say 'quelli cani..' in this case?
Yes, good catch: the adjective plurals of "quello" are "quei" and "quegli". Besides "quelli", though, the only other difference between adjective and pronoun forms is that the latter never get truncated.
To expand on what willem6 wrote, the demonstrative adjective follows the same rules as the definite article, so "il cane" - "quel cane", "i cani" - "quei cani"; the demonstrative pronoun instead declines like a noun, so it's "quello" or "quella" (that, that one) and "quelli" or "quelle" (those, those ones).
So "Quei cani non mangiano formaggio" and "Quelli non mangiano formaggio" are correct, but Quelli + noun is not correct?
Exactly, and it distinguishes e.g. "quegli sciocchi non mangiano formaggio" (those simpletons don't eat cheese) vs "quelli sciocchi non mangiano formaggio" (the foolish ones don't eat cheese). In the first one "sciocchi" is a noun (it answers "who?"), in the second one it's an adjective (it answers "which ones?").
Sorry to be dim but Im not getting this, can someone please break ig down for me?
Those sweets are nice - quei dolci sono belli - Quei is an adjective. I will have one of those - avrò uno di quelli - Quelli is a pronoun