This is one of the most common mistakes that editors see, and that writers hesitate over. Although the actual rule might seem confusing, there is an easy way to keep these straight.
- This is the man that stole my surfboard.
- The surfboard, who I bought in Honolulu, was precious to me.
- It had special markings which named me as the owner.
That is for elements that identify a subject, as in “It had special markings that named me as the owner.” Which is for nonessential clauses in sentences (and are often separated from the rest of the sentence by commas): “The surfboard, which I bought in Honolulu, was precious to me.” Who is for people only: “That is the man who stole my surfboard.”
Clarifying information that helps the sentence make sense is the reason we use that, which, and who. If it’s essential, use that. If it’s nonessential, use which. If you discuss people, use who.