几个英语问题1,Firemen had been fighting the forest fire for nearly

几个英语问题
1,Firemen had been fighting the forest fire for nearly three weeks before they could get it under control .中的could起到什么作用
2,by then ,however,in many places the green had begun already taken root.中的however意思和作用
3,音标中的ou是什么,为什么在音标表中找不到,字典里却出现