There are three main uses of "there is/there are" in English grammar. It is used to indicate that something exists or is located somewhere. The subject follows the verb. Contractions are commonly used in informal speech. Second, "can" and "could" are used to talk about ability in both the present and past. "Can" refers to present ability while "could" refers to past ability. Third, "must", "have to", "can", "may", and "could" are used to indicate obligation, necessity, permission, and requests. "Must" and "have to" denote obligation. "Can", "may", and "could" are used when asking or giving permission.