What, which, and who are often used interchangeably, but each has its special meaning, and they are not synonymous when used at the beginning of clauses in complex sentences.
Where a clause is “restrictive”, or necessary, to the meaning of a sentence and refers to a thing, not a person, it should begin with that. Example: “Please deliver this book to the store that is located on the northeast corner of Main and High Streets.” If the italicized clause is omitted, we have no way of knowing which store is being referred to (assuming there are several stores in the area); the directions will be incomplete without the clause; it is therefore restrictive and requires that.
On the other hand, consider: “Please deliver this book to Border’s Book Store, which is my favorite place to browse.” While the speaker has made the sentence more colorful by describing the store as a favorite, the clause in italics is “non-restrictive”; i.e., it could be dropped from the sentence, and the direction would still be clear (assuming there is only one Border’s in the area).
Note too, another difference between the restrictive and non-restrictive clauses; in the first example the clause beginning with that is not set off by a comma, but in the second example it is. The reasoning is fairly simple (isn’t it always?): if the thought expressed in the clause is essential (i.e., restrictive) to the sentence’s main thought, the comma is omitted to tell the reader this clause is integral; and if a comma is used before the clause, the reader will know that what follows may embellish but could be omitted (i.e., non-restrictive) without putting the essence of the sentence’s thought in jeopardy.
As to beginning a clause with who, the rule could hardly be more direct and simple to apply: never refer directly to a person with a that or which. Correct usage: “He is arriving with Mary, who is his wife.” (Note the use of a comma, since the clause beginning with “who” is non-restrictive.) Incorrect (and all too common): “They are the same players that won the championship last year.” Note, however, that the clause beginning erroneously with that (should have been who) is not set off by a comma since the sentence would not make sense without it, and it is therefore restrictive.
If you never much thought about this previously, blame me. I am the one who called to your attention this mishmash, which can be confusing until you know the rules that govern the use of these three vital pronouns. (Did we get that right?)
Where a clause is “restrictive”, or necessary, to the meaning of a sentence and refers to a thing, not a person, it should begin with that. Example: “Please deliver this book to the store that is located on the northeast corner of Main and High Streets.” If the italicized clause is omitted, we have no way of knowing which store is being referred to (assuming there are several stores in the area); the directions will be incomplete without the clause; it is therefore restrictive and requires that.
On the other hand, consider: “Please deliver this book to Border’s Book Store, which is my favorite place to browse.” While the speaker has made the sentence more colorful by describing the store as a favorite, the clause in italics is “non-restrictive”; i.e., it could be dropped from the sentence, and the direction would still be clear (assuming there is only one Border’s in the area).
Note too, another difference between the restrictive and non-restrictive clauses; in the first example the clause beginning with that is not set off by a comma, but in the second example it is. The reasoning is fairly simple (isn’t it always?): if the thought expressed in the clause is essential (i.e., restrictive) to the sentence’s main thought, the comma is omitted to tell the reader this clause is integral; and if a comma is used before the clause, the reader will know that what follows may embellish but could be omitted (i.e., non-restrictive) without putting the essence of the sentence’s thought in jeopardy.
As to beginning a clause with who, the rule could hardly be more direct and simple to apply: never refer directly to a person with a that or which. Correct usage: “He is arriving with Mary, who is his wife.” (Note the use of a comma, since the clause beginning with “who” is non-restrictive.) Incorrect (and all too common): “They are the same players that won the championship last year.” Note, however, that the clause beginning erroneously with that (should have been who) is not set off by a comma since the sentence would not make sense without it, and it is therefore restrictive.
If you never much thought about this previously, blame me. I am the one who called to your attention this mishmash, which can be confusing until you know the rules that govern the use of these three vital pronouns. (Did we get that right?)
– Ken Butera