1. More generally, the verbs in the hypothetical conditional clauses are backshifted, the past tense form being used for present and future time reference and the past perfective form for past time reference.
一般说来，假设条件句的谓语动词时态是后移的：过去时形式用来指现在时间，过去完成体形式用来指过去时间。
2. A perfective tense used to express action completed in the present.
一种用来表达现在已经完成的动作的完成时态。
3. A tense of verbs used in describing action that has been completed (sometimes regarded as perfective aspect).
一种用来描述已经完成（有时被认为是完成的方面）的动作的动词时态。
4. A perfective tense used to describe action that will be completed in the future.
一种用来描述将来将要完成的动作的完成时态。