Defining relative clauses, also known as restrictive relative clauses, provide essential information about the noun they refer to. This information is crucial for defining or restricting the meaning of the noun.
When the noun refers to a person, ‘who’ is often used as the subject of a defining relative clause. For example, in the sentence “The woman who lives next door is a doctor,” ‘who’ is the subject of the clause and refers back to ‘the woman’. The clause “who lives next door” provides essential information about which woman is being referred to.
On the other hand, ‘that’ can also be used as a subject in defining relative clauses when referring to both people and things. For instance, in the sentence “He’s the man that saw me yesterday,” ‘that’ refers to ‘the man’, and the clause “that saw me yesterday” tells us which man is being referred to.