had he been here yesterday,he would attend.为什么是had了,后面是would do而不是would have done

had he been here yesterday,he would attend.
为什么是had了,后面是would do而不是would have done

这个是错综时间虚拟语气条件句,先说倒装,当虚拟语气的条件句用到had done(对过去）或were（对现在或将来）或should（对将来）时要省略if,把前面的三种情况的任何一种置于句首倒装,你的句子实际是：If he had been here yesterday倒装之后就变成你所写的了,而错综时间是主句的所要表达的时间和从句所要表达的时间不同时,就会产生了,你的句子要表达的是如果过去他怎么样,他现在就会怎么样,所以主句应该要用对现在的虚拟would/should/might/could +V动词原形