by now在句子中通常需要使用什么时态呢？

当表述“到现在为止已成的动作”时，必须使用现在成时。例如：

• They have finished the project by now.
• We have known each other for five years by now. 此类语境中，by now 隐含“动作本应在当前时间前”的语义，因此需用“have/has + 过去分词”结构体现时间截止性。

  若句中存在情态动词表示推测，情态动词+现在成时 是固定搭配。例如：

  • She should have arrived by now.
  • He must have heard the news by now. 这里的“should/must + have done”结构，既by now的时间逻辑，又表达对过去动作的推断。

    特殊语境下，若整个句子为虚拟语气，by now 可搭配过去成时，但仅用于与过去事实相反的假设：

    • If we had left at 7, we would have reached there by now.
    此结构中，by now 虽指向现在，却因虚拟语气框架需用“had + 过去分词”的逆推逻辑。

    需严格区分的是，by now 不可与一般过去时连用。错误示例：

    • She finished the work by now.应为 has finished
    一般过去时仅表示过去某点发生的动作，法体现“持续到现在”的时间跨度，这与by now的核心语义存在根本冲突。

    在时间轴概念中，by now 以“现在”为基准点，谓语动词覆盖“过去起点→现在终点”的整区间，这正是现在成时的语法本质。论是肯定句、否定句还是疑问句，这一原则始终适用：

    • Have you received my email by now?
    • They haven't called by now.