the policeman could not swear to having seen the accused man at the scene of
问题描述：
the policeman could not swear to having seen the accused man at the scene of
为什么是swear to having 而不是to have
答
“swear to sth.” 是一个固定的短语,意思是“断言,肯定,保证”
所以上句中having 是动名词,如果用have的话就错了,因为短语中to后不能加动词
要记住,to 是一个介词,它可以接动词,也可以接名词,虽然一般来说都是动词,但涉及到某些固定搭配,或者表示朝、向某种趋势等时,后面会加名词或动名词,例如：see to sth.办理,照管,料理