- A+
现在完成进行时the present perfect continuous tense一、概念
现在完成进行时表示“从过去某个时间开始到现在一直在进行的动作，这一动作很可能
在这个时间段内未完成”。
二、构成
has been doing
have been doing
三、用法
1 表示从过去某个时间开始一直持续到现在而且尚未完成的动作。例如：
i have been working on this report since eight o’clock this morning. hopefully, i will be
able to finish it by noon.
i’ve been waiting for an hour but he still hasn’t come.
2 表示动作的延续性。例如：
it has been snowing constantly for the last three hours. i hope it stops soon!
laura has been teaching me english for almost two years.
3 表示最近一段时间反复发生的事情。例如：
i have been cycling to work for the last three weeks.
i have been talking to everyone about the problem, but i don’t have a solution. do you have
any ideas?
注意：
非延续性动词和表示状态的动词不用于进行时，所以也不用于现在完成进行时。
例如：
i have wanted to visit china for years.（√）
i have been wanting to visit china for years.（×）
we have understood everything.（√）
we have been understanding everything.（×）
表语从句predicative clauses一、概念
位于系动词之后充当表语的句子叫作表语从句。表语从句用来说明主语的身份、特征、
属性或状态。从句用陈述语序，从句的时态要根据主句时态和句意确定。引导表语从句的有
从属连词that、whether、because等，连接代词what、which、who、whose等，以及连接副词
when、where、why、how等。
二、用法
1 从属连词that、whether、as if（as though）、because等引导的表语从句：
my suggestion is that we (should) start early tomorrow.
what i want to know is whether the enemy is marching towards us.
it sounds as if/though somebody is knocking at the door.
he did not see the film last night. that is because he had to help his little sister with her
homework.
2 连接代词what、which、who、whom、whose等引导的表语从句：
that is what i want to tell you.
the problem is who will take charge of the project.
3 连接副词when、where、why、how等引导的表语从句：
next monday is when we shall meet.
the book is where you left it.
that is why she failed to pass the exam.
the problem is how we can get the things we need.
注意：
系动词与其后的表语一起才能表达一个完整的意义。表示状态的系动词有be、
seem、look、 appear、smell、sound、taste、feel、remain、keep、stay、prove、stand、sit、lie等。例如：
it appears that there has been a mistake.
her head felt as if it would burst.
表示变化的系动词有become、go、grow、get、run、fall、turn等。例如：
he has become what he wanted to be ten years ago.
the troops were ordered to remain where they were.
不定式作主语the infinitive as subject一、概念
不定式或不定式短语可以在句子中作主语，相当于一个名词，谓语动词用单数形式。
二、构成
动词不定式一般由“to + 动词原形”构成。
三、用法
1 不定式或不定式短语作主语时，表语如果是非谓语动词一般也采用不定式形式。例如：
to see is to believe.
to work is to struggle.
2 在很多情况下，为了保持句子平衡，避免头重脚轻，我们常常用代词it代替不定式作主
语，而把不定式移到句子的后面。例如：
it took me half an hour to walk there.
it’s expensive to maintain a car.
it’s difficult to 
open the window.
3 不定式可以用介词for引出其逻辑主语，常用的这类形容词有hard、easy、necessary、
difficult、safe、expensive、impossible、interesting、exciting等。但是如果形容词既表示
不定式的特征又表示人的特征时则一般用介词of引出。常用的这类形容词有kind、nice、
clever、silly、wise、rude、selfish等。例如：
it’s impossible for so few workers to do so much work in a single day.
it was nice of you to take me to the station.
it’s silly of mary to give up the job when she needs the money.
主语从句subject clauses一、概念
在复合句中充当主语的句子被称为主语从句。引导主语从句的有从属连词that、whether
等，连接代词what、which、who、whom、whose、whatever、whichever、whoever等，以及连
接副词when、where、why、how等。
二、用法
1 that引导主语从句，说明一个基本事实，that没有具体含义。这类从句在大多数情况下都放到句子的后面，而用代词it作形式上的主语。例如：
that she won the race did not surprise us.→ it did not surprise us that she won the race.
it was clear that all of them made a mistake.
2 whether引导主语从句，说明一个“不定”的事实，意为“是否”，从句的末尾可以和 or (not) 搭配。例如：
whether we 
will go there tomorrow depends on the weather.
to this day, it’s still unclear whether he shot himself or was murdered.
3 连接代词和连接副词引导主语从句，表示一个以该词为核心的事实，连接词本身有意义，在从句中充当句子成分。例如：
when he will come to visit china again will be discussed later.
why the student was absent from class was the teacher’s first concern.
whoever fails to see this will make a big mistake.
where the bridge will be built is what they are talking about.
what they need most in the flooded area is the supply of food, fresh water and power.
注意：
1 主语从句一般按单数处理，因此，谓语动词常用单数。
2 that和whether引导主语从句，在从句中不作句子成分，一般不能省略。
3 为保持句子平衡，常用it作形式主语，代替真正的主语从句，而将真正的主语
从句置于主句之后。
动词的-ing形式作主语the v-ing form as subject一、概念
动词的-ing形式是动词的一种非限定形式，在句中可以起名词作用，可单独或以短语形式
作主语。
二、构成
动词的-ing形式由“动词原形+ing”构成，其被动形式为“being+过去分词”；完成形式
为“having+过去分词”；否定形式为“not +v-ing”。
三、用法
1 直接位于句首作主语。例如：
reading 
is an art.
climbing mountains is really fun.
2 用it作形式主语，把动词的-ing形式（真实主语）置于句尾作后置主语。例如：
it is fun playing with children.
it is no use/no good crying over spilt milk.
3 用于“there be”结构中。例如：
there is no joking about such matters.
there is no saying when he’ll come.
4 用于布告形式的省略结构中。例如：
no smoking. = no smoking is allowed (here).
no parking.
5 可在动词的-ing形式前面加上一个名词或代词的所有格充当其逻辑主语，构成动词-ing
形式的复合结构。动词-ing形式的复合结构也可以在句中作主语。例如：
the sound of desks being opened and closed could be heard out in the street.
their coming to help was a great encouragement to us.
注意：
1 动词的-ing形式作主语时，通常被看作单数不可数名词，谓语动词要用第三人
称单数形式。
2 动词的-ing形式和不定式都可以作主语。一般来说，在表示比较抽象的一般
行为时多用动词的-ing形式；在表示具体某次动作，特别是将来的动作时，多用不定
式。例如：
reading without thorough comprehension is no good.
it is quite necessary to read it many times.
