1) Do not place a comma before "that"; 2) Do not place a comma before who or which if the information provided by the relative clause is essential; 3) Place a comma before who or which if the relative clause is not necessary to know who or what the speaker means.

When using the relative pronoun "who", "which", or "that" in a sentence:

1. Do not place a comma before "that":

   The man that lives next door is tall.

   The man, that lives next door, is tall.

2. Do not place a comma before "who" or "which" if the information provided by the relative clause is essential to understand which person or thing the writer is talking about.

   The man who lives next door is tall.

   The man, who lives next door, is tall.

   We need the relative clause "who lives next door" to know exactly who is the man the writer is talking about. We do not use commas in a situation like this.

3. Place a comma before "who" or "which" only if the relative clause is not necessary to know who or what you are talking about. That is, the relative clause provides additional information about the person or thing, but this extra information is not essential to identify the subject.

   My neighbor Peter, who lives next door, is very tall.

   Even if you omit the clause "who lives next door", the reader knows who you are talking about (your neighbor Peter). The clause "who lives next door" just adds extra information.

   When someone or something is sufficiently identified, the description that follows is considered nonessential and should be surrounded by commas.

1. Using Commas with Relative Pronouns

Look at this example:

People who live in cities are more stressed.

"Who live in cities" is a relative clause (a part of the sentence that tell us what type of person or thing the speaker means). Relative clauses begin with relative pronouns, such as "who", "which", or "that".

The children who read at home regularly make the most progress.

The clause "who read at home regularly" is essential to understand the meaning of the sentence. If we omit this clause, the sentence makes no sense.

The children make the most progress.

The sentence above is incorrect since we are trying to say that only some students, those who read at home regularly, make the most progress.

Another example:

My brother, who works as a dentist, is wealthy.

If we take out the relative clause ("who works as a dentist"), the sentence is complete and makes sense. It also provides the most relevant information (My brother is wealthy.)

When using a comma before "who" or "which", include also the closing comma (the comma after "dentist" in the example above). Some writers forget to add this important comma. At the end of a sentence, a period replaces a comma.

2. Using a Comma Before 'Who' or 'Which' Changes the Meaning of a Sentence

Examine this example:

My sister, who is very tall, can speak English and French.

This sentence is correct because you are assuming that the reader knows who you are talking about (your sister).

However, if you have two sisters, the reader may need to know which sister you are speaking about. Remember, we should avoid commas if the information is essential to know who or what you are talking about:

My sister who is very tall can speak English and French.

Another example:

I'm planning to go to Spain which is very sunny next month.

In this sentence, the comma is not optional. The clause before the relative pronoun tells us where you are going (Spain). In this particular context, the relative clause "which is very sunny" just adds extra information about Spain.

I'm planning to go to Spain, which is very sunny, next month.

3. When to Use 'That', 'Who' and 'Which'

1. We use "who" for people (sometimes for animals too).

   She doesn't like people who get angry easily.

   I don't like movies who have scenes of violence.

2. We use "which" for things.

   I only buy watches which are waterproof.

   My uncle, which is an engineer, went to London last week.

3. We use "that" for people or things.

   The girl that came last week is smart.

   The dish that you prepared yesterday was delicious.

4. "That" is more common than "which", but sometimes you need to use "which" (see next point).

   Rebecca works for a company that makes drones.

   I don't like movies that have scenes of violence.

5. If the sentence requires commas, you should use "which" for things and "who" for people (not "that").

   Madrid, which is the capital of Spain, was founded around the year 860 A.C.

   Alex, who is very tall, is a good chess player.

6. In many situations, you can omit the pronoun in a relative clause. You can learn more about how to omit correctly that, who, or which.

Remember, we don't put a comma before that.

Paris, that is the most romantic city in the world, is a beautiful place to live.

Paris, which is the most romantic city in the world, is a beautiful place to live.

Another example:

Rebecca, that lives in New York, is very smart.

Rebecca, who lives in New York, is very smart.

4. More Examples

Examples with "who":

• The waiter who served us was rude.
• What was the name of the person who came yesterday?
• The man who was carrying a suitcase was very polite.
• My friend Olivia, who lives next door, is an engineer.
• What was the name of the person who wanted to talk to you?
• The police caught the girl who stole the wallet.
• Marie Curie, who was the first woman to win a Nobel Prize, discovered the existence of the elements polonium and radium.
• The movie was about a boy who was raised in Paris.
• A firefighter is a person who fights fires and rescues people.
• Do you know who took those pictures?

Examples with "that":

• The restaurant that was destroyed in the fire has now been rebuilt.
• The protester that was arrested has hired a new lawyer.
• What happened to the hamburger that was on the table?
• A paradox is a statement or proposition that is seemingly contradictory.
• Andorra is a microstate that borders France and Spain.
• Thailand is the only Asian country that was never colonized by a European country.
• The company that was fined for air pollution is now investing in renewable energy.

Examples with "which":

• Artificial intelligence, which combines computer science and robust datasets, enables the execution of complex tasks.
• The new stadium, which can hold 20,000 people, has an oval shape.
• We often go to Chicago, which is not far away.
• Inflation, which is pretty high right now, is having an impact on US consumers.
• Yoga, which improves core strength and flexibility, is very popular in India.