Van Putte & Garcia (1990) discuss the complementizer-taking preferences of different classes of verbs, based on their occurences in a collection of literary texts. They distinguish between verbs that occur only or mostly with a complement clause introduced by complementizer ku, those with null complementizer, and those which allow both. They assign bisa 'say' to the class of verbs that allow both, while puntra 'ask' is in the class of verbs which occur with the overt complementizer ku, and pidi 'ask, request' prefers a null-complement. They do not, however, distinguish between different modalities. It appears that indicative mood normally requires a ku-complement, with the exception of the complement of bisa or di 'say', where ku is optional.
Kouwenberg & Lefebvre (2007) point out that ku introduces the complement clauses of propositional verbs; these include not only verbs of speaking such as bisa ‘to say’, but also psychological verbs such as realisá ‘to understand’,
sa ‘to know’.
Source: nd: 16 July 2009, p.4
Source: nd: 10 July 2009, p.3
Source: nd: 10 July 2009, p.10