Question

I was + 后的动词如何变化，加原型还是过去式

Answer 1

如果是过去进行时，后面的动词用ing形式
如果是被动语态，后面的动词用过去分词。
如
I was watching TV at nine o'clock last night .

I was told that I had to do my homework again .

Answer 2

I was + 过去分词
构成被动语态，即一把过去时的被动语态。