I think you could buy a new bike.改为否定句

i don't think you could buy a new bike

I don't think you could buy a new bike

I don't think you can buy a new bike.
OR
I think you couldn't buy a new bike.

这里的could不是can的XXX形式，不用被打回原型！
你问的是不是这个意思？？不过我不会解释啊，你看看语法书好了

I don't think you could buy a new bike.
原句为我认为你可以买一个新车
否定句译为我不认为你可以买一个新车
你想问的是不是这个意思？