1. The document discusses modal verbs in English and their uses and forms. It defines modal verbs as verbs used to express ability, duty, necessity, permission or desire.
2. It explains several important differences in how modal verbs are used compared to normal verbs, such as being followed by a base verb form and not taking -s in the third person simple present.
3. The document provides examples of many common modal verbs like can, may, must and will and their meanings and substitutes that can be used in other tenses where the modal verb cannot. It emphasizes the need to understand the meanings and substitutes for proper usage.