but,however,and的英文用法,急!

There used to be someone who I could have loved。There used to be someone who I could have bought posies for。There used to be someone who I could have asked 'Do you love me?'I'm pretty sure that the answer would have been 'yes,I do' 。However / But now there is no one,no one at all。But/However都表示转折,可以互换,However语气强一些。用could have done形式表达更好一些,表示原本能够去做,但是实际上没有做。