Knights always tell the truth,liars always lie,and knaves alternate between truths and lies.
Suppose you ask a native,"Are you a knight,a liar,or a knave?"
The native answers,and you are able to figure out what he is.
What is the native?
The native is a KNAVE.
The only way we can figure out the type of native correctly is if the answer to the question is "A LIAR" and this answer could only be given by a KNAVE and none other.