Van Putte & Garcia (1990) assign sa 'know' to the class of verbs that allows both ku-introduced and null-introduced complements, while lubidá 'forget', like most psych-verbs, occurs only with ku-complements.
Kouwenberg & Lefebvre (2007) point out that ku introduces the complement clauses of propositional verbs; "verbs of knowing" are in this class.
Source: nd: 16 July 2009, p.4
Source: nd: 22 June 2009, p.7
Source: nd: 16 July 2009,p.4