That, Which, Who, Whom, and Whose

The words that, which, who, whom, and whose are called relative pronouns. A relative pronoun can introduce a relative clause: a type of subordinate clause that modifies a subject or object from earlier in the sentence. You can use a relative clause to combine two simple sentences into one complex sentence.

Who refers only to people. Use who to replace a subject pronoun with a relative clause.

Simple sentences: Professor Suarez teaches history. She was born in Bolivia.
Combined: Professor Suarez, who was born in Bolivia, teaches history.

Whom also refers to people, but it is used to replace an object pronoun.

Simple sentences: The Beatles were a British band. Fans idolized them.
Combined: The Beatles were a British band whom fans idolized.

Use whose to replace a possessive pronoun with a relative clause.

Simple sentences: I climbed an old tree. Its branches reached over a stream.
Combined: I climbed an old tree whose branches reached over a stream.

That vs. Which 

As a general rule, that can only be used in restrictive clauses. A restrictive clause restricts the meaning of what it modifies; it is essential to the meaning of the sentence. A restrictive clause is not set off by commas.

The book that was on my desk is now missing.
Experiments that involve toxic chemicals require careful preparation.
A noun is a word that names a person, place, or thing.

Who and Which can be used with restrictive clauses or non-restrictive clauses. NOTE: Some writers prefer to use which only with non-restrictive clauses. A non-restrictive clause is a clause that does not provide essential information and is set off by commas.

Restrictive: They wrote a report that impressed the boss.
Non-Restrictive: The report, which impressed the boss, found ways to increase revenue.
Restrictive: David was the first person who asked me to eat sushi.
Non-restrictive: David, who once lived in Japan, asked me to eat sushi.

Note how the presence or absence of commas can change the meaning of a restrictive clause:

Unclear: College students, who have jobs, tend to manage time more efficiently.
Improved: College students who have jobs tend to manage time more efficiently.

That vs. Who 

That refers to animals and things, and can replace a subject or object. That can be used to refer to people, but who is more humanizing.

Exercise 1: Combine the two simple sentences into one complex sentence using a relative pronoun (that, which, or who) and a relative clause.

Professor Martins teaches math. He was born in Nigeria.
He designed a computer application. It helps students understand calculus.
The application is easy to use. It runs on most computers.
Students take his class. They can download it for free.

Exercise 2: Complete the following sentences with that, which, who, whom, or whose.

Andrea is the kind of scientist __________ enjoys teaching.
She created an exhibit __________ illustrates how mountains are formed.
It was inspired by a student __________ she wanted to help.
The student __________ problem inspired the project was really struggling.
Her exhibit, __________ is in the library, has helped many geology students.