Defining relative clauses

Subject or object

The relative pronoun can define the subject or the object of the verb:
They’re the people who/that bought our house. (The people bought our house. The people is the subject.)
They’re the people who/that she met at Jon’s party. (She met the people. The people is the object.)
Here are some cells which/that show abnormality. (Some cells show abnormality. Some cells is the subject.)
Here are some cells which/that the researcher has identified. (The researcher has identified some cells. Some cells is the object.)

No relative pronoun

We often leave out the relative pronoun when it is the object of the verb:
They’re the people she met at Jon’s party.
Here are some cells the researcher has identified.
See also:

• 
  No relative pronoun
  

Punctuation

Warning:
In writing, we don’t use commas in defining relative clauses:
This is a man who takes his responsibilities seriously.
Not: This is a man, who takes his responsibilities seriously.

Nouns and pronouns in relative clauses

When the relative pronoun is the subject of the relative clause, we don’t use another personal pronoun or noun in the relative clause because the subject (underlined) is the same:
She’s the lady who lent me her phone. (who is the subject of the relative clause, so we don’t need the personal pronoun she)
Not: She’s the lady who she lent me her phone.
There are now only two schools in the area that actually teach Latin. (that is the subject of the relative clause, so we don’t need the personal pronoun they)
Not: There are now only two schools in the area that they actually teach Latin.
When the relative pronoun is the object of the relative clause, we don’t use another personal pronoun or noun in the relative clause because the object (underlined) is the same:
We had a lovely meal at the place which Phil recommended. (which is the object of the relative clause, so we don’t need the personal pronoun it)
Not: We had a lovely meal at the place which Phil recommended it.