Defining relative clauses

Punctuation

In writing, we use commas around non-defining relative clauses:
Etheridge, who is English-born with Irish parents, replaces Neil Francis, whose injury forced him to withdraw last week.
Spoken English:
In speaking, we often pause at the beginning and end of the clause:
Unlike American firms – which typically supply all three big American car makers – Japanese ones traditionally work exclusively with one maker. (formal)
And this woman – who I’d never met before – came up and spoke to me. (informal)

Defining or non-defining relative clauses?

Sometimes defining and non-defining relative clauses can look very similar but have different meanings.
See also:

• 
  Relative pronouns
  

Punctuation

In writing, we use commas around non-defining relative clauses:
Etheridge, who is English-born with Irish parents, replaces Neil Francis, whose injury forced him to withdraw last week.
Spoken English:
In speaking, we often pause at the beginning and end of the clause:
Unlike American firms – which typically supply all three big American car makers – Japanese ones traditionally work exclusively with one maker. (formal)
And this woman – who I’d never met before – came up and spoke to me. (informal)

Defining or non-defining relative clauses?

Sometimes defining and non-defining relative clauses can look very similar but have different meanings.

The soldier who had gold stripes on his uniform seemed to be the most important one.	A defining relative clause which we can’t leave out; without this information we do not know which soldier the speaker is referring to.
The tour party was weakened when Gordon Hamilton, who played in the World Cup team, withdrew yesterday because of a back injury, which kept him out of the Five Nations Championship.	Non-defining relative clauses which we can leave out: The tour party was weakened when Gordon Hamilton withdrew yesterday because of a back injury.

Warning:
We can use that instead of who, whom or which in defining relative clauses, but not in non-defining relative clauses:
I think anyone who speaks in public is nervous beforehand.
I think anyone that speaks in public is nervous beforehand.
Her car, which was very old, broke down after just five miles.
Not: Her car, that was very old, broke down after just five miles.
See also:

• 
  Relative pronouns
  
• 
  Relative clauses: typical errors
  
• 
  Look at example sentence