1、The boy is so young that he can't go to school.
The boy is __not_ old __enough__ to go to school.
2、I went there so that I could see an old friend of mine.
Iwent there __in__ __order__ __to__ see an old friend of mine.
Iwent there _so__ _as___ _to___ see an old friend of mine.
1.enough如果修饰名词：enough + n.(eg:I've enough food.)
enough如果修饰形容词：a.+enough (eg:The room is big enough to live in.)
2.so that = in order that,它们后面都要跟句子.
so as to do= in order to do,它们后+动词不定式,即to do sth.