1. The document discusses modal verbs in English and their uses and properties. Modal verbs such as can, may, must express abilities, obligations, permissions and other meanings.
2. Modal verbs are followed by a base verb form and don't take third person 's' endings. Negation uses "not" without "do". Many modal verbs can't be used in all tenses.
3. The document explains the meanings and uses of various modal verbs like can, may, should, must and semi-modals like have to. It provides examples and substitutes for modal verbs that can't be used in some tenses.