请教一个问题

是关于时态的问题：

混用时态可以吗？什么情况下可以混用时态？（一般情况下，同一句子时态保持一致，即过去时态对应过去时态，现在时态对应现在时态）

谢谢NN的指点！

alohabrian可以举个例子吗？谢谢

ghgriffin谢谢你的回答！

从句也可以 前后时间不一致也可以 尤其注意比较里的时态问题

还有科学事实/规律在从句里的不一致

有好几个比较经典的例子 我刚正好看到一个

82. (28855-!-item-!-188;#058&004055)
 
In a blow to those who still harbored the illusion that E-mail exchanges are private, a watchdog group recently uncovered a trick for enabling an interloper to rig an E-mail message so that this person will be privy to any comments that a recipient had added as the message is forwarded to others or sent back and forth.
 
(A) who still harbored the illusion that E-mail exchanges are private, a watchdog group recently uncovered a trick for enabling an interloper to rig an E-mail message so that this person will be privy to any comments that a recipient had added
(B) who had still been harboring the illusion that E-mail exchanges are private, a watchdog group recently uncovered a trick for enabling an interloper to rig an E-mail message so that this person was privy to any comments that a recipient might have added
(C) who still were harboring the illusion that E-mail exchanges are private, a watchdog group recently uncovered a trick enabling an interloper to rig an E-mail message so that this person is privy to any comments that a recipient would add
(D) still harboring the illusion that E-mail exchanges are private, a watchdog group recently uncovered a trick that enables an interloper to rig an E-mail message so that this person will be  privy to any comments that a recipient might add
(E) still harboring the illusion that E-mail exchanges had been private, a watchdog group recently uncovered a trick that will enable an interloper to rig an E-mail message so that this person was privy to any comments that a recipient might add

还一个OG里的好句子

A 1972 agreement between Canada and the United States reduced the amount of phosphates that municipalities had been allowed to dump into the Great Lakes.
 
A) reduced the amount of phosphates that municipalities had been allowed to dump
 
B) reduced the phosphate amount that municipalities had been dumping
 
C) reduces the phosphate amount that municipalities have been allowed to dump
 
D) reduced the amount of phosphates that municipalities are allowed to dump
 
E) reduces the amount of phosphates allowed for dumping by municipalities

谢谢！