As formas verbais "am" "is" ou "are" pertencem ao verbo to be, o equivalente a ser ou estar em português

i am -  aplica-se à primeira pessoa do singular
he, she, it is - aplica-se à terceira pessoa do singular
you, we, ou they are - aplica-se à primeira, segunda ou terceira pessoa do plural

i am
you are
he, she, it is
we are
you are
they are

i am at school
you are my brother
he is my friend
she is my cousin
we are sleeping
they are swimming