DEFINING RELATIVE CLAUSE

A defining relative clause  describes the preceding noun in such a way as to distinguish it from other nouns of the same class. For instance,
The man who is walking with a stick is my uncle.
In this sentence' who is walking  with a stick is the relative clause.If we drop or remove it we don't know which man in particular we are talking about.Secondly, there is no comma between the noun[the man]and the defining relative clause[who is walking with a stick]
1[a]Using WHO\ THAT for PERSONS in defining relative clauses
The boy who  return within 5 minutes gets first prize.
All who \that heard her sing  were bewitched by her voice.
2 Object of a verb;whom ,or who ,that.WHOM is very formal
The boy whom I saw horseriding is our neighbour.
3WHOM or THAT  with a preposition
The girl to whom I spoke is our class monitor.
4 Possesive; whose
Here is the boy whose article has won  the first prize.
5Using relative pronouns which ,that,whose refer to animals and to things
The new light bulb that you fitted doesn't work.
6When and Where refer to a time or place in the main clause;
Maggie returned to the town  where she had spent her childhood.
REDUCED RELATIVE IN DEFINING CLAUSES
A sentence in which a relative pronoun can be omitted
The boy I wanted to see is absent today.
7Possesive; whose /of which+a clause
The book  the preface of which is by my teacher has been well written.