(2) a. They know [ who to invite. ]
b. They know [ which topic to talk about. ]
c. * They know [ who to speak. ]
Intended meaning: who is the subject of speak.
Here are the structures for the three sentences:
(2c) is ungrammatical because the subject position in which who must originate is not case-licensed, nor is the spec,CP position to which it moves. In principle, the moved who might get case from the matrix verb know but if it did then movement of wh- words from case-licensed positions might be problematic, since they would have to be checked for case twice. For this reason, we assume that verbs like know don't have a case to license when they take CP clausal complements.