Who, That or Which?
The English language is a living and constantly changing language. The following is a snapshot of some of the generally accepted rules for the use of the relative pronouns, "who," "that," and "which." A caveat: some English language mavens have somewhat different interpretations of the rules, so you have to be guided by your choice of the rules.
It is important to remember that "who," "that," and "which" used in clauses are substitutes for persons, places, things or ideas referred to earlier in a sentence (hence, called ’relative pronouns'). The following is just a cursory view of the general rules; there are other uses and other relative pronouns (whom and whose), but this blog post does not include them.
The first general rule is "who" refers to people or animals with names, and "that" and "which" refer to things or groups.
Example #1: Mary is the girl who won the race.
Example #2: My friend is on the team that almost went to the Little League World Series.
In order to explain the use of "which" and "that" used to introduce a clause in a sentence, we have to distinguish between a restrictive or essential clause (necessary for meaning and follows "that") and a nonrestrictive or nonessential clause (supplies added information and follows "which").
Example #3: Essential clause — The team that wins all its games will be the champion.
Example#4: Nonessential clause— The road, which is scenic, is covered with snow.
Note: a nonessential clause is set off with commas. This type of clause adds information but doesn't affect the meaning of the sentence.
Some modern linguists use "which" in restrictive clauses, so don't feel too comfortable or smug about your choice of "that" in restrictive clauses, some will not agree with your choice.
Example #5: Essential clause — The bike which I keep in the garage is ideal for short trips. (Common in British English)
As stated earlier in this post, the so-called language experts differ on the use of "which" in restrictive (essential) clauses, so you have to choose your style and realize that language is shifting beneath our feet. The first principle of Descriptive Linguistics should settle the issue — if it sticks around, it is right.
