Full Idea
Sentences or assertions can be derivately called true, if they succeed in expressing determinate propositions. A sentence can be ambiguous or vague or paradoxical or ungrounded or not declarative or a mere expression of feeling.
Gist of Idea
To be true a sentence must express a proposition, and not be ambiguous or vague or just expressive
Source
David Lewis (Forget the 'correspondence theory of truth' [2001], p.276)
Book Reference
-: 'Analysis' [-], p.276
A Reaction
Lewis has, of course, a peculiar notion of what a proposition is - it's a set of possible worlds. I, with my more psychological approach, take a proposition to be a particular sort of brain event.