This document discusses modal verbs, focusing on should and could. It explains that modal verbs are special verbs that express ability, possibility, necessity or obligation of a main verb. Should is used to give advice, such as "you should go to the doctor." It has no conjugation. Could expresses ability or possibility, such as "you could go to the movies." Both should and could are followed by a base form of the main verb. Their negative forms are shouldn't and couldn't. More information on their uses can be found in Top Notch 1 and at the provided link.