but能不能用在句首?

原文:http://mp.weixin.qq.com/s?__biz=MzA4MDYyMTcxOA==&mid=2651256076&idx=1&sn=0552170548705eb2f74b8a9603799ca5&chksm=84532cf5b324a5e383ee866d959228013e60c5de3dab07ccc5885dac4865383b5952dda91c64#rd