A comma is usually not needed when a sentence includes an extra phrase beginning with what word?

A comma is usually not needed when a sentence includes an extra phrase beginning with the word "that" if the phrase is essential to the meaning of the sentence.

Here's why:

* Essential Phrases: These phrases provide crucial information that defines the noun they modify. Without them, the sentence's meaning would be incomplete or unclear.

* Non-Essential Phrases: These phrases offer additional information but don't change the fundamental meaning of the sentence. They can be removed without affecting the core idea.

Examples:

* Essential: "The book that I borrowed from the library is overdue." (Without the phrase, we don't know which book is overdue.)

* Non-Essential: "The book, which I borrowed from the library, is overdue." (We already know which book is overdue, so the phrase provides extra information.)

Important Note: There are exceptions to this rule, and sometimes a comma is necessary even with "that" phrases. It's best to consider the specific sentence and its meaning.