I have always been confused with the difference between a priori and a posteriori so here goes my way of distinguishing them:
Something is knowable a priori if it can be known by a person without experiencing it. Something is knowable a posteriori if you have to experience it in order to know it. For example, if you say that all bachelors are unmarried, that is a priori. If you say that it is raining outside now, that is a posteriori.
A person who knows (a priori) that “All bachelors are unmarried” doesn’t have to experience bachelorhood in order know it. On the other hand, if I say that “It is raining outside,” I should have a previosu experience with rains in order to know for sure that it is indeed raining.