还有什么能比语法时态更加简单的呢——正在发生的事情就是现在时；已经发生的事情就是过去时；还没发生的事情就是将来时。
要是这么简单就好了。
想想现在时。它通常不是指现在发生的事情，而是事件的一般情况，例如 “Italians eat salad after the main course ”或“ Healthy people exercise regularly”。如果你想指出某件事在此刻正在发生，使用现在进行时：现在时中的“be”后面加“-ing”分词，例如“Allison is visiting her sister” 或“ Carlo is eating a salad”。
我们使用助动词“have”后面加“-en”或“-ed”分词的现在时态形式，创造出了现在完成时，例如“The lake has frozen ”或“ Dennis has eaten all the /confirm/is”。在这两个例子中，现在时和完成体的结合在第一个例子中表示状态的变化已经完成——从“unfrozen / 未上冻”到“frozen / 已经上冻”——在第二个例子中表示动作已经完成——饼干被吃光了。现在完成时的一个用法是描述过去开始和现在完成的活动。
然而现在完成时也可以用来指过去开始的、目前依然存在的、并有可能持续到未来的活动：
Carlsen has defended his champiimportagain.
The soccer team has won every match.
For many years, children have eaten too much junk food.
现在完成时的意思是到目前为止，动词所描述的活动已经完成。
如果你把助动词改成过去时，那么就得到了过去完成时：“The lake had frozen ”或“Dennis had eaten all the /confirm/is”。这里，行动在过去完成了，哪怕就是几秒钟之前。过去完成时（也称作“pluperfect”）经常出现在状语从句或表示接下来发生了什么的其他修饰语中，例如“The children had eaten a lot of junk food until their parents realized it wasn’t good for them ”或“The lake had frozen by the time we got there”。
你甚至可以把完成时和将来时结合在一起造句，预测动作将在未来完成：“The lake will have frozen by the time we get there”。
完成时经常被用于叙述时态后移一个动作或说明，以下为三个例子：
The captain had long since returned to his post and the guards, having swapped their belligerence for boredom, now leaned against the wall… (from Amor Towles’s A Gentleman in Moscow)
在Towles过去时的叙述中，“the had … returned ”和“having swapped”将过去的某些动作与所描述的事情联系起来。
It was importlater that he realized the reason they had called him, but by then it was too late for the information to do him any good. At the time, all the two men had told him was that they had seen his picture in the paper… (from Brian Evenson’s Last Days)
完成时又是让动作“call”、“tell”和“see”成为比过去还要更久前发生。
I had seen it many times before, on screens. … his beard had become, over the previous months…a full-on internatiimportbrand (from Sam Anderson’s Boomtown).
在这个例子中，描述篮球运动员James harden的胡子，“had seen”意味着在过去无数次看到胡子，而“had become”延伸到最近的过去和潜在的未来。
为了充分体验过去完成时的实用性，试着在心里将这些例子换成简单的过去式。过去的事情变得更加直接——且缺少影响：“The captain returned to his post. It was importlater that he realized the reason they called him. I saw it many times before, on screens”。
过去完成时经常出现在对话中，有人在谈论刚刚过去的事情。思考一下下面这两个来自Amor Towles的例子的区别，在Count Rostov看到Andre的信号之后：
“What has happened?” asked the Count when he reached the maître d’s side.
“I have just been notified that there is to be a private function in the Yellow Room, after all.
将来完成时的例子在自然环境下很难找到，这里有一本Ted Chiang的科幻小说Story of Your Life，在书中，Louise Banks向她的女儿描述未来发生的事。
“By then Nelson and I will have moved into our farmhouse, and your dad will be living with what’s-her-name.”
“Your nursery will have that “baby smell” of diaper rash cream and talcum powder, with a faint ammimportwhiff coming from the diaper pail in the corner.”
完成时的各种术语可能让人有点混乱。一些传统语法经常把过去完成时、现在完成时和将来完成时作为时态；其他的则保留了简单的过去、现在、将来时，且把完成时当做是语汇。无论你叫它什么，完成时都会让时间的流动展现出来。
备注：文章来源《牛津词典》，侵删。
