第四单元环境和我们。
13、清洁的自来水被用来洗脸、刷牙、洗衣、拖地后就成了污水。
21、血液中的细胞好像运输兵，负责运输吸入的氧气和产生的二氧化碳。
答：说明米饭不是甜的，但米饭含有淀粉，在我们咀嚼的过程中发生了变化，变得有甜味了。
4、咀嚼馒头的外皮也可以感觉到甜味吗？为什么？
8、铁生锈的原因是什么？人们怎样防止铁生锈？
2、昆虫种类繁多，分布很广，它们有着和其他动物不同的身体构造和本领。
答：当月球运行到地球和太阳的中间，如果月球挡住了太阳射向地球的光，便发生日食。
答：①可以节约能源；②减少对环境的污染；③降低成本。
8、晶体的形状多种多样，但都很有规则。有的是立方体，有的像金字塔，有的像一簇簇的针……有的晶体较大，肉眼可见，有的较小，要在放大镜或显微镜下才能看见。句子成分。
英语的句子成分主要有六种：即主语、谓语、宾语、定语、状语和补语。（可以熟记为：
主谓宾，定状补）除了这六种主要成分之外，还有“表语”和“同位语”的说法。但表语和系动词一起作谓语，因此划分成分时，划分在谓语上。同位语分为主语同位语和宾语同位语，属于主语或宾语的一部分。
主语。主语是一个句子所叙述的主体，一般位于句首，通常由名词性的词来充当。
可以作主语的词性或语法结构：
1.名词 2.代词 3.数词 4.名词化的形容词（如the rich）5.不定式 6.动名词 7.主语从句等表示。
during the 1990s, american country music has become more and more popular.
we often speak english in class.
one-third of the students in this class are girls.
to swim in the river is a great pleasure.
smoking does harm to the health.
the rich should help the poor.
when we are going to h**e an english test has not been decided.
it is necessary to master a foreign language.
that he isn’t at home is not true.
改正下列句中的错误，并说明原因。
he failed the exam is the reason why he dropped out.
that why he was late for school was that his mother was ill.
beyond the mountains lie a small village.
gone is the days when i had to go to school on foot.
play basketball is my f**orite sport.
give up english is not an option.
谓语。谓语由动词充当，说明主语所做的动作或具有的特征和状态。谓语的构成如下：
简单谓语：由一个动词或动词短语构成。如：
he practices running every morning.
he reads news*****s every day.
复合谓语：由情态动词或其他助动词加动词构成。如：
you may keep the book for two weeks.
he has caught a bad cold.
my sister is crying over there.
i h**e been waiting for you all the time.
i would stay at home all day.
由系动词加表语构成。系动词不能单独作谓语，要和表语一起作谓语。如：
we are students.
your idea sounds great.
表语。表语多是形容词，用以说明主语的身份、特征和状态，它一般位于系动词（如be, become, get, look, grow, turn, seem等）之后。表语一般由名词、代词、形容词、分词、数词、不定式、动名词、介词短语、副词及表语从句表示。
our teacher of english is an american.
is it yours?
the weather has turned cold.
the speech is exciting.
three times seven is twenty one.
his job is to teach english.
his hobby is playing football.
the machine must be under repairs.
the truth is that he has never been abroad.
宾语。宾语由名词性的词充当，表示动作的对象或承受者，一般位于及物动词和介词后面。
宾语分为动词宾语和介词宾语，分别构成动宾结构和介词结构。
they planted many trees yesterday.
(how many dictionaries do you h**e?) i h**e five.
they helped the old with their housework yesterday.
i wanted to buy a car.
i enjoy listening to popular music.
i think（that）he is fit for his office.
宾语补足语。
英语中有些及物动词，除有一个直接宾语以外，还要有一个宾语补语，才能使句子的意义完整。宾语补足语和宾语构成逻辑上的主谓关系。换句话说，在意思上，宾语相当于宾补的主语。
带有宾语补足语的一般句型为：某些及物动词（如make等）+宾语+宾补。宾补可由名词、形容词、副词、不定式、分词、介词短语和从句充当。
his father named him dongming.
they painted their boat white.
let the fresh air in.
you mustn’t force him to lend his money to you.
we saw her entering the room.
we found everything in the lab in good order.
we will soon make our city what your city is now.
i want your homework done on time.
主补。对主语的补充。含有宾语补足语的句子在变成被动语态，宾语作主语时，原来的宾补就成了主语补足语。
he was elected monitor.
she was found singing in the next room.
定语。定语是对名词或代词起修饰、限定作用的词、短语或句子，汉语中常用“……的”表示。定语通常位于被修饰的成分前。在英语中，许多情况下，定语是放在所修饰词后面的。
the next man is a scientist.
1.形容词短语作定语一般放在所修饰词之后。
the man next to me is a scientist.（我旁边的那个人）
2.副词用作定语一般要后置。
people there are very friendly. (那儿的人们)
he didn’t like the man downstairs. （楼下的那个人）
3.介词短语作定语时要后置。
the boy under the tree is tom.（树下的那个男孩）
the tallest boy in our class is john.（我们班最高的那个男孩）
4.现在分词短语、过去分词短语、动词不定式作定语常后置。
i h**e something to say. (我有要说的话)
the boy crying over there is my classmate.（在那边哭的那个男孩）
the house built last year is impressive.（去年建的那座房子）
the letter on the desk is for mr. wu.
the woman with a baby in her arms is his mother.
we need a place twice larger than this one.
she carried a basket full of eggs.
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