Question

______ him not to do so,he wouldn't have made such a serioue mistake.

A Did I persuade
B If I persuade
C If I should persuade
D Had I persuade

Answer 1

这里考查的虚拟时态的用法，一眼就知道答案选 D。
细看后半句用的是wouldn't have done sth，这说明句子的前半句一定是发生在过去，省略if，had提前。
Had I persuaded him not to do so,he wouldn't have made such a serioue mistake.
=If I had persuaded him not to do so,he wouldn't have made such a serioue mistake.

Answer 2

C