There are three main uses of "there is/there are" in English: to indicate existence or location of something, to use in contractions, and to indicate plurality. The subject follows the verb when using "there is/there are".
Can and could are used to talk about ability, both present and past. Can is used for present ability and general skills, while could is used for past ability. Must and had to indicate obligation or necessity, with must for present and had to for past.
Can, could, and may are used when asking for permission, with can being more casual and could/may being more formal. Can is also used when giving permission.