多数情况两者都可以.按理说,应该用主格+助动词或系动词,如：
He studies harder than I do.
I feel better than I did yesterday.
但是在一些简单的结构中,宾格也都可以用.类似上面的第二句话就不能用宾格.如：
She is taller than I.
She is taller than me.