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Identifying a person with essential extra information.
Why you specifically
The rule
Defining relative clauses with who give essential information about a person. The clause tells us which person we mean, and who is the normal relative pronoun for people in this structure. In defining relative clauses, the information is necessary, so we do not use commas.
The trap
The man which lives next door is very friendly.
The man who lives next door is very friendly.
Who is the defining relative pronoun for people. It links the extra information to the person.
I know a teacher which speaks three languages.
I know a teacher who speaks three languages.
Who is used for people, not things.
She is the woman that helped me yesterday.
She is the woman who helped me yesterday.
The relative clause must use who to describe a person in the subject position.
We need a guide knows this city well.
We need a guide who knows this city well.
Who is correct when the relative clause identifies a person. Using a noun alone without a relative clause does not link the description properly.
Prove it
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