“That” and “which” are pronouns used to introduce clauses in a sentence. If you don't know where to use them carefully you will be confused a lot. The following information will help you to overcome these problems while using "which" and "that".
• Our customers were confused by the instructions. + They were not written very clearly. = The instructions, which were not written very clearly, confused our customers.
• The article was clearly plagiarized. + It was removed from the publication. = The article that was clearly plagiarized was removed from the publication.
The confusion sets in when it comes to deciding which pronoun to use—“that” or “which.” They are not interchangeable. And they should certainly never be mixed for the sake of word variation.
One way of deciding whether to use “that” or “which” is to determine if the clause in question can be omitted without changing the meaning of the sentence. If the clause can be omitted, use “which.” If the clause cannot be omitted, use “that.”
“The article that was clearly plagiarized was removed from the publication.”
“That” is used in this sentence because “that was clearly plagiarized” tells the reader which article was removed: The one that was plagiarized.
“The instructions, which were not written very clearly, confused our customers.”
In this case, “which” is used in this sentence because you can remove the clause “which were not written very clearly” without changing the meaning of the sentence. Another way to remember which word to use—always use “that” unless you could justifiably place a comma before the clause. “Which” always mandates the use of a comma.