Questions on Locke's Epistemology

1. Why would you think that there is a difference between a table as it appears to you and the "real" table as it is in itself?
2. How do empiricists (e.g., Locke) differ from rationalists (e.g., Leibniz) about how we know that it is impossible for something both to be and not be?
3. How does Locke know that his ideas of sense are caused by external objects?