除了 (chú le) is a preposition meaning except, besides, or apart from. It has two distinct uses. With 以外/之外 and a negative verb (除了你以外，我谁都不信, I trust nobody except you) it means except. With 还 or 也 and a positive predicate (除了学习，他还喜欢运动, besides studying, he also likes sport) it means in addition to or besides. Context and the following predicate determine which meaning applies.