Final answer:
Clauses starting with 'that' are usually essential to the meaning of a sentence, and therefore should not be separated by commas. Non-essential information is set off by commas, whereas lack of commas around a clause indicates that the clause is essential to the sentence's meaning.
Step-by-step explanation:
Clauses beginning with that are typically essential to the meaning of the sentence, and therefore, should not be set off by commas. Non-essential information, which can be removed without changing the basic meaning of a sentence, is usually set off by commas. Conversely, essential information is integral to the meaning of the sentence, and lack of commas indicates that the information cannot be readily omitted without altering the sentence's meaning. A comma serves as a mark of separation in a sentence, signifying nonessential information and aiding the reader in understanding the structure and nuances of a sentence.
For example, in the sentence 'The book that has a red cover is mine,' the clause 'that has a red cover' is essential information because it specifies which book is being referred to. Removing that clause would change the meaning of the sentence because we would no longer know which book is mine. On the other hand, in the sentence 'The library, which was built in 1923, is on Main Street,' the clause 'which was built in 1923' is non-essential and is properly set off by commas, as its removal does not change the fact that the library is on Main Street