• Choose which to set off something nonrestrictive (nonessential to the meaning of the sentence) -- or, as we say, parenthetical.
Choose that to set of something restrictive (essential) -- something you wouldn't put in parenthesis:
Nonrestrictive: The Nile, which flows into the Mediterranean, gives Egypt life.
Restrictive: The Nile is the river that gives Egypt life.
• If you think of a nonessential clause as something parenthetical -- an aside -- you can remember that which introduces a clause set off by parentheses, dashes or commas.
That introduces a clause not set off by parentheses, dashes or commas.
Restrictive: The policy that critics charged was flawed from the beginning was amended.
[tells which policy of several]
Nonrestrictive: The policy, which critics charged was flawed from the beginning, was amended.
[merely adds a fact parenthetically about the policy under discussion]
Restrictive: The house that had a brick front was theirs.
[tells which house]
Nonrestrictive: The corner house, which had a brick front, was theirs.
[merely adds a fact parenthetically about the house]
• Use what rather than that or which mainly in questions and in place of the phrase that which or those which:
What book has Democrats seeing red these days?
[question]
Pundits say he stands a good chance to get what he wants.
[meaning that which]
Next week, we'll talk about the differences between who, whom, whoever and whomever.
Choose that to set of something restrictive (essential) -- something you wouldn't put in parenthesis:
Nonrestrictive: The Nile, which flows into the Mediterranean, gives Egypt life.
Restrictive: The Nile is the river that gives Egypt life.
• If you think of a nonessential clause as something parenthetical -- an aside -- you can remember that which introduces a clause set off by parentheses, dashes or commas.
That introduces a clause not set off by parentheses, dashes or commas.
Restrictive: The policy that critics charged was flawed from the beginning was amended.
[tells which policy of several]
Nonrestrictive: The policy, which critics charged was flawed from the beginning, was amended.
[merely adds a fact parenthetically about the policy under discussion]
Restrictive: The house that had a brick front was theirs.
[tells which house]
Nonrestrictive: The corner house, which had a brick front, was theirs.
[merely adds a fact parenthetically about the house]
• Use what rather than that or which mainly in questions and in place of the phrase that which or those which:
What book has Democrats seeing red these days?
[question]
Pundits say he stands a good chance to get what he wants.
[meaning that which]
Next week, we'll talk about the differences between who, whom, whoever and whomever.