时态(注意未划线部分的时态)
1，过去完成时表示过去的过去，何时使用：
(1)强调过去事件的顺序
(2)for past actions that were ongoing until, or relevant to, a later past action ;
(3)for past actions that were ongoing until, or relevant to, a later past TIME MARKER.
(4)单独的had done, had been done是不能单独出现的，必要条件之一就是要出现一个过去的动作和a later past time maker。
2，(1)现在完成时：发生在过去的动作持续到现在;动作在过去已经结束，其影响持续到现在。但不意味着动作当前正在进行，也不是动作以后也会进行。只是强调过去到现在。
Stacy关于现在完成时的解说：
I like to think of present perfect as something that bridges past and present, and it's either still going on or just simply still true today.
eg, I have been to Argentina
I'm not still there today, but it is still true today that I went there at some point. (Not really - I've never been to Argentina! And look what I did there - I used present perfect again, because it was true in the past and is still true today. I have never been there.)
Another example:
The results of the study have shown that the more we study, the smarter we get.
The study is over - but the results are still true today, as indicated by usage of the present perfect tense.
以下情况用现在完成时：
l within/during/in/over/for+ the past/recent/last+时间段.
注意in last与in the last的区别：
e.g. : Did you see the game on TV last night? 昨天晚上
e.g. : Interest in golf has grown rapidly in the last ten years. 最近十年
l 出现since考虑现在完成时
since的四种用法:
(1) since +过去一个时间点(如具体的年、月、日期、钟点、1980, last month, half past six)。
I have been here since 1989.
(2) since +一段时间+ ago
I have been here since five months ago.
(3) since +从句
Great changes have taken place since you left.
Considerable time has elapsed since we have been here.
(4) It is +一段时间+ since从句
It is two years since I became a postgraduate student.
OG的解释：the past tense nor the present perfect is correct; both suggest that the statue’s features once constituted an artificial face but no longer do so.
to have dong/ comma+having done/ .will do that sb(sth) have done
以上三种完成时都表示动作发生在另一个动作之前。
Whether tourists will still come to the park once the animals’ horns have been trimmed.
The Neanderthals appear to have been equipped to face any obstacle
The scientist, having......, does sth.
Appear的用现在是表示对present-day observers，用过去时就是对过去的observer
(2)以下情况用一般过去式：
l 出现previously/originally, first
l 过去明确时间。如in 1971
l 过去段时间。如between 1897 and 1900
以下情况用一般现在时
l 自然现象
l 统计数据
l 实验结论(科研成果)
l 客观事实，经常性事件
(3)一般过去时表示动作在过去已经结束了，而过去进行时就表示动作当时还在继续进行，并没有结束。
(4)时态混用。主句的动作已经结束，但是内容还在进行。主要有科研成果，政府决策，协议内容
3，将来时：
(1)条件从句/时间从句/方式从句/让步从句不能用将来时或过去将来时，常用一般现在时代替一般将来时，一般过去时代替过去将来时。(对比状从省略的几种是：让步，时间，条件。)
(2)possibility that…/likelihood that…/expect that…中,that从句后用一般将来时
(3)标准书面语中,优先使用一般将来时,主观将来时用be supposed to do,较少用be going to do,不用be to be doing/be to be done