12. 选 D。根据句中的 didn’t let me drive 可知，这是一个与过去事实相反的虚拟语气句，条件句的谓语用 had+过去分词，主句谓语用 would / should / could / might have+过去分词。

13. 选 C。as if 引导的句子，有时用虚拟语气，有时不用。原则上说，若指事实或可能为事实，不用虚拟语气；若指现在，从句谓语动词用一般过去时(be 用 were)；若指过去用过去完成时，若指将来用过去将来时或用 were to do sth。又如下面一题要选 C：

It seems as if the sun ____ round the earth since it rises in the east and sets in the west.

A. circles                                                  B. is circling

C. were circling                                        D. has been circling

14. 选C。but for 的意思是“若不是，要不是”，用于引出与事实相反的假设，通常与虚拟语气连用。根据句中的 the help you gave me，可知它表示与过去事实相反，故选C