Grammar

8.90 referring to people

When you are referring to a person or group of people, you use 'who' or 'that' as the subject of a defining clause. 'Who' is more common than 'that'.

The man who employed me would transport anything anywhere.
...the people who live in the cottage.
...somebody who is really ill.
...the man that made it.

You use 'who', 'that', or 'whom' as the object of a defining clause, or you do not use a pronoun at all.

...someone who I haven't seen for a long time.
...a woman that I dislike.
...distant relatives whom he had never seen.
...a man I know.

You use 'that' as the complement of a defining clause, or you do not use a pronoun.

...the distinguished actress that she later became.
That was the kind of person she was.

After a superlative, you do not usually use a pronoun.

He was the cleverest man I ever knew.
...the best boy we ever had.

For more information about superlatives see paragraphs 2.117 to 2.127.