RESTRICTIVE CLAUSE--THAT A restrictive clause is just part of a sentence that you can't get rid of because it specifically restricts some other part of the sentence. Examples: "Cars that have hybrid technology get great gas mileage." "Gems that sparkle often elicit forgiveness." The words "that have hybrid technology" or "that sparkle" restrict the kind of gems you're talking about. Without them, the meaning of the sentence would change. Without them, you'd be saying that all gems elicit forgiveness, not just the gems that sparkle. NONRESTRICTIVE CLAUSE--WHICH A nonrestrictive clause is something that can be left off without changing the meaning of the sentence. You can think of a nonrestrictive clause as simply additional information. Here's an example: "Diamonds, which are expensive, often elicit forgiveness." "There was an earthquake in Japan, which is bad news." Here, leaving out the words "which are expensive" or "which is bad news" doesn't change the basic meaning of the sentence. (Also note that the phrases are surrounded by commas. Nonrestrictive clauses are usually surrounded by, or preceded by, commas.)

that可引导 主语/宾语/定语/表语/同位语从句；which可引导 宾语/定语/表语/同位语从句；what可引导 主语/宾语/表语/同位语从句。 that在主语/定语（that作从句的主语)/表语/同位语从句中绝对不能省。