As it goes, there are knights, who always tell the truth, and knaves, who always right. Suppose, a question is that there are a knight and a knave, and we have to find out if a Statement S is true or not  .
So, instead of asking:
"What would the other guy reply if I asked if S is true ?" Or something like that
What's wrong, if I ask simply:
If you're a knight, then answer "Is S true ?" and if you're a knave, then reply "Is S false ?"
Then, (If I don't make any dumb mistake), that would tell the truth value of S.
What's wrong with that (If this is true, then this should answer any knight-knave puzzle like in condition X, there are knight and knaves, and you pick n randomly, and ask them yes/no questions to find truth about S ?
0: S has no reference to knight/knaves, just casual ones like "This island contains treasure", "X is the correct maps", not meta ones like "This statement is false" or "I am a knave" (His head will explode if he says it)