他们试图想出一个能够提前完成任务的计划.
by是介词,关系代词紧跟在介词之后时,不能用that,指物时只能用which,指人时只能用whom.而且不能省略.
但如果介词移到后面,则可以用that.而且,由于介词后面的成分叫宾语,关系代词作宾语时,只要不是紧跟在介词之后就可以省略.
上例可以说成：
They tried to think of a plan which/that they could fulfil their task by ahead of time.
或They tried to think of a plan they could fulfil their task by ahead of time.