Use of the copula is restricted to a small class of adjectives, most of which are relatively recent borrowings from Dutch, such as enthoesiast ‘enthusiastic’, bezig ‘busy’, ernstig ‘serious’, vrij ‘free’, etc. (Winford 1997: 283)
The majority of predicate adjectives CANNOT occur with a copula.
See our comments on Features 8 "Order of degree word and adjective" and 42 "Comparative standard marking".
Source: Winford 1997: 282
Source: Winford 1997: 238
Source: Winford 1997: 238
Source: Winford 1997: 283
Source: Winford 1997: 283