Joanie warned Henry that she was an expert pool player.
Henry is the direct object.
The clause is required for discourse coherence.
Unlike the ditransitive communication verb to tell, verbs that take clausal complements are more or less deontic; they always express the fat that the subject has affected or is attempting to affect the behavior of the direct object in some way.
convinced sb that ...
assured sb that...
advised sb that...
cautioned sb that ......
persuaded sb that .......
warned sb that ......
