1.当用于同级比较时,as .as 既可用于肯定中也可用于否定句中,
而so.as 只能用于否定句中.
His pencil is as long as mine.
His pencil isn't so/as long as mine.
2.当表示只要时,两者没有什么区别,可以替换.
e.g.Our profits will be good as/so long as the dollar remains strong.