When you place an order at the market price that gets filled immediately, you are considered a taker and will pay a fee of 0.5%
When you place an order which is not immediately matched by an existing order, that order is placed on the order book. If another buyer / seller places an order that matches yours, you are considered the maker and your fee will be 0%.
When you place an order that gets partially matched immediately, you pay a taker fee for that portion. The remainder of the order is placed on the order book and, when matched, is considered a maker order. The fee is 0% for that part of the total order.