1.翻译"No sooner have we begun talking than you came in."
2.have been doing是现在完成进行时,表一个从过去持续到现在的动作,强调进行状态；have done是已经完成.eg：We have been studying here for nearly 3 years. We have studied here since three years ago.
3.现在完成时的过去式,通俗地讲就是“过去的过去”,这时就用had.eg：In my youth,I had ever been Peking.(只在小时候的过去）