为什么I后面能接were 也能接was

一般情况,I后都接WAS等单数形式
If I were a boy.此句型为虚拟句型,后接主语+SHOULD形式
在虚拟句型里,不管从句主语是什么,都是主语+WERE的形式
When I was ten years old ...这句为一般句型,即时间状语从句,所以用WAS
既然看到你问了,我就在这里补充一下虚拟语气的问题：
非真实条件句
1）时态：可以表示过去,现在和将来的情况.它的基本特点是时态退后.
a.同现在事实相反的假设.
句型 ：条件从句 主句
一般过去时 should( would) +动词原形
If they were here,they would help you.
b.表示于过去事实相反的假设.
句型：条件从句 主句
过去完成时 should(would) have+ 过去分词
If she had worked harder,she would have succeeded.
The rice would not have been burnt if you had been more careful.
If my lawyer had been here last Saturday,he would have prevented me from going.
If he had come yesterday,I should / would have told him about it.
含义：He did not come yesterday,so I did not tell him about it.
If he had not been ill and missed many classes,he would have made greater progress.
含义：He was ill and missed many lessons,so he did not make greater progress.
希望我的回答令你满意