Answer:
When you are describing a human subject (the person doing or being something in the sentence), an adjective clause begins with who or that.
Step-by-step explanation:
An adjective clause is a group of words containing a subject and a verb. It offers further description or information - similar to what an adjective does - about something that was mentioned in another sentence. Adjective clauses are often introduced by relative pronouns such as who, which, when, where, whose or that.
If the clause is describing a human subject (the person doing or being something in the sentence), it must begin with either "who" or "that". If the clause is describing a non-human subject, it must begin with either "which" or "that". Since what is important in this question is the human subject, let's take a look at the examples below:
1. Look over there! That is the man who sold me a counterfeit watch! --> "who" refers to "man". "Man" performed the action of selling.
2. My sister is the girl that always wears a read sweater. --> "that" refers to "sister". "Sister" performs the action of wearing.