"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。