A nonrestrictive clause is a relative clause that introduces additional information that may be omitted. Restrictive clauses simply elaborates on a noun and the information it introduces is not necessary. Restrictive clauses are delimited by commas, unlike restrictive clauses.
For instance, in the following sentences:
- I wanted to thank personally your mother, who helped me this last week.
- The book, which quickly became popular, has sold more than a million copies.
- He just met his brother, John Smith, after 20 years of hard search.
…the bold parts indicate the nonrestrictive clause.
Don't use "that"
The relative pronoun that can be used in restrictive clauses, but do not use it in nonrestrictive clauses:
- Correct: The book, which quickly became popular, has sold more than a million copies.
- Incorrect: The book, that quickly became popular, has sold more than a million copies.