When we use a relative pronoun as a subject in the relative clause, we don’t use a personal pronoun or noun:
That’s the schoolthatdoes lots of music and drama.
Not:That’s the school that it does lots of music and drama.
When a relative pronoun is the object of the relative clause, we need a subject (pronoun or noun) in the relative clause:
They met at the gallerythatJanehad talked about.
Not:They met at the gallery that had talked about.
When a relative pronoun is the object of the relative clause, we don’t need another object (pronoun or noun) in the relative clause:
They went to the same restaurant that Mark had been to.
Not:They went to the same restaurant that Mark had been to it.
In writing, we don’t use commas in defining relative clauses:
Sally is a committee member who finds it difficult to make decisions.
Not:Sally is a committee member, who finds it difficult to make decisions.