Explain the difference between defining and non-defining relative clauses.

Defining and non-defining relative clauses are two types of clauses used to provide additional information about a noun or pronoun in a sentence. However, they differ in terms of their essentiality and punctuation.

Defining relative clauses, also known as restrictive relative clauses, are used to define or identify the noun or pronoun they modify. These clauses provide essential information that is necessary to understand the meaning of the sentence. Without a defining relative clause, the sentence may lose its intended meaning or become ambiguous. Defining relative clauses are not separated by commas and are closely connected to the main clause.

For example:
- The book that I bought yesterday is really interesting.
In this sentence, the defining relative clause "that I bought yesterday" specifies which book is being referred to. Without this clause, the sentence would simply state "The book is really interesting," which lacks specificity.

Non-defining relative clauses, also known as non-restrictive relative clauses, provide additional information about the noun or pronoun they modify but are not essential for understanding the main point of the sentence. These clauses are separated by commas and can be removed from the sentence without changing its core meaning. Non-defining relative clauses often provide extra details or explanations about the noun or pronoun.

For example:
- My sister, who is a doctor, lives in London.
In this sentence, the non-defining relative clause "who is a doctor" provides additional information about the noun "my sister." However, if we remove this clause, the sentence still conveys the main point that "My sister lives in London."

In summary, the key difference between defining and non-defining relative clauses lies in their essentiality and punctuation. Defining relative clauses are necessary for understanding the sentence's meaning and are not separated by commas, while non-defining relative clauses provide extra information but can be omitted without altering the core message and are set off by commas.