Sheldon : If I could, I would, but I can’t, I shan’t
翻译：如果我可以那么我会做，但是我不能所以我不会做。
shall
1）表征询意见，用于第一、第三人称疑问句。
Shall I get you some tea? 我给你点茶好吗？
Shall the boy wait outside? 让那男孩在外面等吗？
What shall we do this evening? 我们今晚做什么？
2）表说话人的意愿，有“命令、允诺、警告、决心”等意思，用于第二、第三人称陈述句。
You shall do as I say. 按我说的做。（命令）
You shall have my answer tomorrow. 你明天可以得到我的答复。（允诺）
He shall be sorry for it one day, I tell you. 有一天他会后悔的，我告诉你。（警告）
Nothing shall stop us from carrying out the plan. 什么也不能阻止我们执行这项计划。（决心）
Gandalf: You shall not pass!!! (你这科过不了)
should
1）表义务。意为“应该”（某件事宜于做），用于各种人称。
You should be polite to your teachers. 你对老师应该有礼貌。
You shouldn t waste any time. 你不应该浪费时间。
2）表推测，意为“想必一定、照说应该、估计”等。
The film should be very good as it is starring first—class actors.
这部新电影是一流演员主演的，估计拍得很好。
They should be home by now. 照说他们现在应当已经到家了。
情态动词 should 表推测时，意为“想必会，理应……”但与“have +过去分词”连用时，则又可意为“本应该做某事却没做”。例如：
She should have attended your birthday party, but she had to look after her mother in hospital. 她本该出席你的生日晚会的，可是她得在医院照顾她妈妈。
Tom should not have told me your secret, but he meant no harm.
汤姆本不该告诉我你的秘密，可是他并无恶意。
时态绕口令：
Season 8 episode 5
完成时都强调对事件完成之后的时间点的影响。
Howard： In 2015，Biff steals the sports almanac and takes the time machine back to 1955 to give it to his younger self.
But as soon as he does that, he changes the future. So the 2015 he returns to would be a different 2015, not the 2015 that Marty and Doc were in.
Leonard： This is 《Hot Tub Time Machine》all over again.
If future Biff goes back to 2015 right after he gives young Biff the almanac, he could get back to the 2015 with Marty and Doc in it. Because it wasn’t until his 21st birthday that 1955 Biff placed his first bet.
Sheldon：
Wait.Whoa.Whoa. Is “placed” right?
Leonard： What do you mean?
Sheldon：Is “placed” the right tense for something that would have happened in the future of a past that was affected by something from the future?
Leonard： Had will have placed?
Sheldon：That is my boy.
Leonard：Okay, so, it wasn’t until his 21st birthday, that Biff had will have placed his first bet and made his millions. That’s when he altered the timeline.
Sheldon： Yeah, but he had will haven’t placed it!
Howard： What???
Sheldon： Unlike Hot Tub Time Machine, this couldn’t be more simple. When Biff gets the almanac in 1955, the alternate future he creates isn’t the one in which Marty and Doc Brown ever use the time machine to travel to 2015. Therefore, in the new timeline, Marty and Doc never brought the time machine…
Leonard：W-Wait. Is “brought” right?
Sheldon： Marty and Doc never had have had brought?
Leonard： I don’t know. You did it to me.
Sheldon： Oh, I’m going with it. Marty and Doc never had have had brought the time machine to 2015. That means 2015 Biff could also not had have had brought the almanac to 1955 Biff. Therefore, the timeline in which 1955 Biff gets the almanac is also the timeline in which 1955 Biff never get the almanac. And not just “never gets”.Never have, never hasn’t, never had have hasn’t.
Raj： He is right.
时态和语法分析：
在英语时态中，“时“指动作发生的时间，”态“指动作的样子和状态。
Howard： In 2015，Biff steals the sports almanac and takes the time machine back to 1955 to give it to his younger self.
But as soon as he does that, he changes the future. So the 2015 he returns to would be a different 2015, not the 2015 that Marty and Doc were in.
这里Howard，使用的都是一般现在时
Leonard： This is 《Hot Tub Time Machine》all over again.
If future Biff goes back to 2015 right after he gives young Biff the almanac, he could get back to the 2015 with Marty and Doc in it. Because it wasn’t until his 21st birthday that 1955 Biff placed his first bet.
好戏开始：这里的placed是Leonard的想法，他认为在《回到未来2》中1955年发生的事情就是过去发生的事情，而一般现在时不强调对现在（2015）造成的影响,于是Leonard认为placed是没有问题的。
但是时空穿越是Leonard没有考虑的
Sheldon：
Wait.Whoa.Whoa. Is “placed” right?
此时Sheldon提出了疑问
Leonard： What do you mean?
Sheldon：Is “placed” the right tense for something that would have happened in the future of a past that was affected by something from the future?
他认为，placed是不恰当的，因为placed bet的Biff的这一行为对未来造成了影响，就不能使用一般过去时。
他的这句话的翻译就是：一般过去时是一种可以用来表示过去某个被将来影响的时间点的将来发生的事情吗？
Leonard： Had will have placed?
此时，这句话就变成了：
Because it wasn’t until his 21st birthday that 1955 Biff had will have placed his first bet.
根据这个表格，leonard的这句话是两种时态的融合，过去完成时，和将来完成时 had will have done. 这个动作是过去将来完成的。
had done 表示这个动作是过去完成的,表示在过去某个时间点之前这个动作以及完成。
will have done 表示这个动作是未来的某个动作，且这个动作在未来某个时间点之前会完成。
时间点是他21岁生日，时间段的事件是买彩票这个动作。
此时，old Biff会处在活在两条时间线上，一条是旧的时间线，一条是新的时间线,但是对于他本人来说，他的生命是线性的。
对他来说1955年和1956年既是过去也是未来，也就是说此时的时态要可以提现出他在过去和将来同时完成，也就是”过去和将来同时完成时”
Sheldon：That is my boy.
Leonard：Okay, so, it wasn’t until his 21st birthday, that Biff had will have placed his first bet and made his millions. That’s when he altered the timeline.
Sheldon： Yeah, but he had will haven’t placed it!
Howard： What???
Sheldon： Unlike Hot Tub Time Machine, this couldn’t be more simple. When Biff gets the almanac in 1955, the alternate future he creates isn’t the one in which Marty and Doc Brown ever use the time machine to travel to 2015. Therefore, in the new timeline, Marty and Doc never brought the time machine…
Leonard：W-Wait. Is “brought” right?
Sheldon： Marty and Doc never had have had brought?
根据这句话的含义，young biff 拿到 almanac的时候 Marty已经出发但是他们在旧的时间线。此时在新的时间线中的未来未来的他不能的到时光机，也就是说在新的时间线中Biff没有再穿越回来。
此时就会出现四个关键时间点new 2015 , old 2015, new 1955, old 1955,
Marty出发后young biff 1955的到almanac，此时在新的时间线中对于old biff 来说 Marty never had brought almanac to 2015
但是这个行为对现在old biff 1955来说的，就是说此时new time line 中的Marty 1955已经出发于是可以说是have brought
在这个句子中的现在时old biff 1955和young biff 1955传递almanac的时候。
但是在对于Old biff来说，old 2015和old 1955是同一个时间点，因为Marty时空穿越。所以此时还有一个had brought
所以对old biff 来说就是有3个时间点，都是在“过去”，第一个时间点是old 1955 但是由于时间穿越他和old 2015重叠，还有一个是new 1955，于是就有三个过去。
old 1955
old 2015
new 1955
所以整句话就成了：
never had(old 1955) have(new 1955) had(old 2015) brought
从来没有已经带来过
Leonard： I don’t know. You did it to me.
Sheldon： Oh, I’m going with it. Marty and Doc never had have had brought the time machine to 2015. That means 2015 Biff could also not had have had brought the almanac to 1955 Biff. Therefore, the timeline in which 1955 Biff gets the almanac is also the timeline in which 1955 Biff never gets the almanac. And not just “never gets”.Never have, never hasn’t, never had have hasn’t.
Raj： He is right.