"by" 前面动词的时态取决于具体的语境和表达方式。一般来说,现在时态(如一般现在时、现在进行时、现在完成时等)比较常见,因为它们能够表达当前的状态或动作。例如:
I usually go to school by bus. (一般现在时)
He is studying by the window. (现在进行时)
They have finished the work by Friday. (现在完成时)
但是,过去时态(如一般过去时、过去进行时、过去完成时等)也可以使用,以表达过去的状态或动作。例如:
She used to go to school by bike. (一般过去时)
They were playing by the river at that time. (过去进行时)
We had finished the work by Monday. (过去完成时)
此外,将来时态(如一般将来时、将来进行时、将来完成时等)也可以使用,以表达未来的状态或动作。例如:
They will come back by train. (一般将来时)
She will be studying by the window tomorrow. (将来进行时)
They will have finished the work by next Friday. (将来完成时)
总之,"by" 前面动词的时态需要根据具体的语境和表达方式来选择,但一般包括现在时态、过去时态和将来时态。