现在完成时是由have/has+v过去分词那么变一般疑问句是为什么要把have和has提前做句首.不是说陈述句便一般疑问句时,有be动词提前,没有时加助动词do或does吗?

变一般疑问句的规则是有be动词时,提前be 动词；没有时提前助动词.
比如一般现在时：
He likes English.变为 Does he like English?
They go to school by bus everyday.变为 Do they go to school by bus everyday?
现在完成时中have/has是助动词.所以变一般疑问句时要提前助动词have和has,在这里它们没有实际意义,不表示“有”的意思.只是辅助形成完成时态.也就是说,它们之于现在完成时的作用相当于do/does之于一般现在时的作用.例如:
They have already lived here for three years.变为
Have they already lived here fro three years?