The reason for the confusion?
Those who try to explain their use invariably launch into an explanation using terms such as relative pronouns, subordinate clauses, restrictive clauses, and nonrestrictive clauses. If you’re not asleep after this explanation, then you’re more confused than ever.
I am now going to explain the use of “which” and “that” without using any of these sedating, bewildering terms. But I warn you, this is a tough one. Keep your wits about you.
“That” and “which” are pronouns used to introduce clauses in a sentence. Their use allows writers to combine sentences and avoid choppy prose. For example:
• 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.