【问】 Waiting for a plane to take off from an airport can often take so long as the trip itself.(TOEFL，74.3)

我查了许多词典，都注明as long as=so long as。为什么上面的试题要求将so long as改为as long as?

【答】 当as long as和so long as作“只要”解时，可以看作是同义的(If you say that one thing istrue as long as or so long as another thing is true，you mean that it is true if or when the otherthing is true.)例如：

1. You can go out，as long as you promise to be back before 11 o’clock.(LDAE)

只要你答应在11点以前回来，你就可以出去。

2. So long as you make Ruth happy I’m content. (Walpole)

只要你使露丝幸福，我就满意了。

但是，当用于同级比较时，在肯定句中只能使用as long as，而不能使用so long as。

1. You can stay here as long as you want. (Hemingway)

你希望在这里呆多久就呆多久。

2. This river is as long as that one.

这条河与那条河一样长。

上面的试题之所以要将so long as改为as long as，就是因为上面的原因。

(参见as...as/so...as)