Quote Originally Posted by zorro View Post
Throbert--So in this pair of sentences, Борис пожаловался, что ему скучно./Борис пожаловался на то, что ему скучно., на то is not required, but there are other times it is required, it that right?
Correct! However, I'm not sure if there's an easy and general rule-of-thumb about when the на то would be required, and when it would be optional. It mainly depends on the specific verb that's used in the main clause, before the subordinate "что" clause.