by now 不等于 now。

by now 在很多情况下，表示过去时间，类似于before now。用一般过去时是常态。例如：

By now the plane was running out of fuel.  

现在，这架飞机的燃料快用完了。

来自柯林斯例句

By now it was completely dark outside.  

这会儿外面全黑了。

来自柯林斯例句

Manchester United were by now oozing with confidence.  

曼彻斯特联队现在充满信心。

来自柯林斯例句

当然，by now 还可用于其他时态。

        by now 还可用于现在完成时。例如：

       I hope you've understood by now what I mean. (LDCE) 我希望现在你已经明白我说的意思了。