(1) The teachers will explain the grammar to you.
(2) I just wanted to comment that in English people cannot say:
He is a [one] my brother's teachers.
So English speakers have invented
the construction (kind of sentence): He is a [one] teacher of
(3) Maybe in some other languages, you can say:
She is a Robin's friend. But not in English. It has to be:
She is one of Robin's friends/ She is a friend of Robin's.
Three extra ideas:
(A) If you say "He is a teacher of my brother," that is "good"
English, but it is not idiomatic (the way native speakers use the
(B) Some books say that "He is a teacher of my brother's" =
He is a [one] teacher of my brother's [many teachers].
(C) Sometimes you may not use the 's:
A picture of my father's. = A [one] picture of my father's [many pictures
that he bought].
A picture of my father. = The picture shows my father's face.
If you have time, search this website and the Web for articles about
the so-called double genitive.