┣┣下面这3句话哪里错了啊

应该改成
They have been best friends for a long time.
因为是for a long time, 是一个状态,因此不能用瞬间动词become
It will be rainy the day after tomorrow.
这里的it指天气,而不能用there
It's raining outside. You'd better stay at home.
had better do something是固定搭配,中间没有to喔~
加油~ 呵呵~

1 become 为短暂动词 不可以跟for+一段时间
They have been best friends for a long time
2 there be 句型表示存在，后一般接名词 表天气用It作主语
It will be rainy the day after tomorrow
3 had better do sth. 固定搭配 =will do well to do sth.= may/might as well do sth.
It's raining outside.You'd better stay at home.

been become为瞬间动词
there-it
to删掉 You'd better ＋原型

become_been
rainy_rain
to stay_stay