When to use the pronouns which and that to commence a clause is a challenge to many, and their misplacements are common.
A clause can be either independent (“I went to the store” – – which is a complete thought) or dependent (“When you arrive…”) which is incomplete and needs something to complete the thought. Most clauses beginning with either which or that are dependent, and our goal is to guide you in the use of each.
This is easily stated: that should be used only when the clause is restrictive, and which should be used only when the clause is non-restrictive. A clause is restrictive when its integral to the thought that is being expressed.
Consider:
“The fire destroyed the barn; the cows that slept in the field survived.”
“The fire destroyed the barn; the cows, which slept in the field, survived.”
The first sentence implies that only the cows sleeping in the field survived, and those in the barn did not. The second sentence implies that all the cows survived and, incidentally, they were sleeping in the field.
In the first sentence the clause beginning with that is restrictive; i.e., it is absolutely essential to complete the meaning of the sentence. In the second sentence, the clause beginning with which is non-restrictive. It is only an incidental observation and could be omitted without altering the meaning, which is that all of the cows survived.
More illustrations of restrictive clauses:
The police were directed to go to the house that was at 101 Main Street.
The student bought the book that she needed for her math class.
Non-restrictive:
“Johnny played baseball with his glove, which was four years old.”
“The sun, which had risen at 6:30 am, was occluded by clouds throughout the day.”
Note that neither of the sentences with restrictive clauses makes sense if the clause is removed as an essential bit of information would be missing; but the non-restrictive clauses, though they may add to the color and texture of the sentences, can easily be deleted without affecting the main thought.
Note also that the restrictive clause (that) should never be set off by commas since it is an integral part of the sentence; conversely, the non-restrictive clause (which) should always be set off by commas.
Another concern is the placement of the clause; it should always be nestled up against the noun or pronoun it is modifying. Failure to do so often leads to ambiguity. “The police chased the getaway car for 20 miles, which was traveling at times more than 100 miles per hour.” The clause should be placed immediately after “car” as it was moving at 100 mph; as written it modifies “miles” which makes no sense. Or, “The dog chased the cat around the yard, which had brown fur.” Which animal had the brown fur; for certain the yard, which it modifies, has no fur?
Finally, an absolute proscription: Never begin a clause that refers to a person with anything but who (or whom where appropriate – – another issue). “We called the doctor who was on call.” “The pass went to the receiver to whom ten passes had already been thrown.” This is a rule easily stated but often ignored, as speakers often use the pronoun that to refer to people.
— Ken Butera