In
A Comprehensive Grammar of the English Language (1985), Quirk et al. have a section called "
[E3] Adjective complementation by a wh-clause" (pp. 1225-1226), where they cite the adjectives sure, unsure, unclear, obvious, doubtful, plain, careful, etc., as adjectives that can be followed by a -
wh clause with or without an intervening preposition. Here are some of their examples. The parentheses around the prepositions indicate that the preposition is optional:
- I was unsure (of/about) what I should say.
- I was unsure (of/about) whether the problem was solved.
- John is careful (about) what he does with his money.
- Are you sure (of) how much the machine costs?
- I wasn't altogether clear (about) what we had to do.
About their featured adjectives, they write: "Most of these adjectives are intrinsically negative in meaning. In other cases, . . . the adjective tends to occur with a wh-clause in nonassertive contexts" (p. 1225). Incidentally, I have chosen not to list examples of theirs like "It was unclear whether an amendment would be accepted," because such examples can be analyzed as
it-extraposition, that is, as deriving from "Whether an amendment would be accepted was unclear."