Essential phrases (also known as restrictive or defining clauses) are phrases that provide essential information to identify the noun they modify. They are necessary for the sentence to make sense and are not set off by commas.
Example:
* "The book that I borrowed from the library is due tomorrow."
The phrase "that I borrowed from the library" is essential because it tells us which specific book is due tomorrow. Without this phrase, the sentence would be unclear.
Non-essential phrases (also known as non-restrictive or non-defining clauses) provide additional information about the noun they modify, but are not essential to its identification. They are set off by commas.
Example:
* "The book, which I found at a used bookstore, is a classic."
The phrase "which I found at a used bookstore" is non-essential because it provides extra information about the book. The sentence would still be understandable without this phrase.
In summary:
* Essential phrases are not set off by commas.
* Non-essential phrases are set off by commas.