I don't understand use of there is and there are

There is - when after "there" ,the coming noun is singular or uncountable. For ex:- There is a boy in this class. There is some water in that bottle. There are - when after "there", the coming noun is plural. Ex:- There are three girls in this area. In both situations, "there" is used as a dummy subject which has no mean.It's only used for completing this type of sentence.