The verb haber is used in two different ways. The first is in perfect tenses, which you can check out here. The other use is the express “there is/are”.
It will be conjugated only one way per tense. In present indicative tense it is “hay”. Notice that the verb doesn’t change between singular and plural.
Hay un libro. → The is a book.
Hay muchos libros. → There are many books.
Hay un mono. → There is a monkey.
Hay muchos estudiantes. → There are many students.
Hay que means “to have to”. Although it is similar to tener que, you only use it when you are not clarifying who needs to do the action.
Tienes que poner la mesa. → You need to set the table.
Hay que poner la mesa. → The table must be set.
Hay que comer. → It is necessary to eat.