您正在看的高中二年级是:Saving the earth。 ost boys like to swim once they have learned how.男孩一学会游泳，多半都喜欢上了。

　　Once you hesitate, you are lost.一犹豫，你就完了。

　　Once you begin, you must continue.一旦开始你就得继续下去。

　　Once you have take the examination, you’ll be able to relax. 一旦考过，你就可以轻松一下了。

　　▲ Once可作“一次，一回”解

　　I saw him once and and never again.我见过他一回，以后再也没见过。

　　I’m going to tell you once.我只能告诉你一回。

　　3. Cattle are once cause of the problem. In good years, farmers increse the numbers of their cattle.造成戈壁滩的原因之一是牲畜问题。年景好时，农民就增加牲畜饲养的头数。

　　▲cattle意为“牲畜”，主要指牛，亦可作“羊”解，是复数。如 cattle and sheep牛和羊，不可说 a cattle，而要说one head of cattle 二十头牛说 twenty head of cattle， head of在指多头牛时可省去。例：

　　He has twenty (head of) cattle on the farm.他在农场养了二十头牛。

　　Cattle were allowed to graze on the village common.牛群被允许在村里的公共用地上吃草。

　　These cattle are from Holland.这些牛来自荷兰。

　　Rearing cattle is an important branch of agriculture.畜牧业是农业的重要组成部分。

　　【注】养牛既可说 keep cattle，也可说rear cattle ，如 keep a large herd of cattle意为“养一大群牛”，贩卖牛(牲口)，可说 trade/deal in cattle或 carry on a trade in cattle

　　▲ Cause此处作名词，意为“原因，起因”，是可数名词。例：

　　We can’t get rid of war unless we get rid of the causes of war.除非我们能去除战争的根源，否则我们不能消灭战争。

　　What was the cause of these misunderstandings?这些误解的原因是什么?

　　The cause of the fire was a cigarette end.火灾的起因是一个香烟头。

　　No one could guess the cause.没人能猜出原因。

　　▲ Cause可用作动词，意为“引起，导致”，后可接双宾语。

　　The operation caused the patient much pain.手术给病人造成了很大的痛苦。

　　I’m afraid I am causing you much trouble.我担心我给你增添了许多麻烦。

　　▲ Cause可接不定式作宾补。如：

　　The lightning caused the baby to cry..闪电把孩子吓哭了。

　　What caused him to change his mind?是什么使他改变了主意?

　　4. Then one year, the rains fail to come.The farmers don’t want to kill any of their cattle,so the cattle eat all the grass and anything that grows on poorer land.然后有一年没有下雨。农民又不想把牲畜杀掉，所以牲畜就把草以及更加贫瘠的土地上长出的任何东西都吃掉了。

　　▲fail是动词，意为“没有能够”，表示“没有能够做某事”。常用 fail to do sth.

　　She didn’t work hard at her lessons so she failed to pass all the exams.他不努力学习功课，所以各门功课考试都不及格。

　　I have just failed to see Watt.He was out.我没能见到瓦特。他出去了。

　　He did very well,but he failed to break the record.他表现得很出色，但没有破纪录。

　　He didn’t fail to keep his word.他没有忘了遵守诺言。

　　The car failed to climb the hill.汽车没能爬上小山。

　　▲ Fail后面也可接名词或代词，意为“使失望，无助于”等。例：

　　His courage failed him in the end.最后，他失去了勇气。

　　His friends failed him when he most needed them.当他最需要朋友时，他们却使他失望了。

　　The teacher failed me on the written paper.老师给我的笔试卷打了不及格。

　　▲ Fail也可作不及物动词用，意为“失败，倒闭，身体变弱”等》

　　When money is in short supply, many businesses fail.当缺乏资金来源时，许多企业就要倒闭。

　　He has been failing in health for the last two years.近两年来，他的体质每况愈下。

　　His eyesight is failing.他的视力正在下降。

　　Last year the crops failed.去年庄稼歉收。

　　5. If this continues for several years,the grass never grows again and the land becomes a desert.One suggested answer is for farmers to limit the numbers of their cattle.这种情况如果继续几年，那么，草就再也长不出来，土地也就变成了沙漠，有人提出解决的办法是让农民限制饲养牲畜的数量。

　　▲suggested是过去分词作定语，修饰 answer，动词is后是 不定式的复合结构作表语用，其中farmers是不定式 to limit the numbers of their cattle的逻辑主语。

　　▲ Desert在这里是名词，意为“ 沙漠”，“一片沙漠”是 a desert，多片沙漠是 deserts。在一般情况下，desert前要有定冠词the,介词用in。

　　Few kinds of plants grow in the desert.仅有几种植物在沙漠上生长。

　　It’s a story about an Arab in the desert.这是一个生活在沙漠地区的阿拉伯人的故事

　　An Arab was walking alone through the desert when he met two men.一个阿拉伯人在独自穿越沙漠时遇到了两个人。

　　6. Another problem is that good soil is gradually lost.When trees are cut down,there is nothing to hold the soil in place on the hillside.另外一个问题就是沃土逐渐流失，树木被砍伐了，山坡上就没有什么东西能够把土壤保持住了。

　　▲ Hold在此处意为“保持住”，相当于 keep，其后常跟复合宾语，可有形容词，副词，介词短语等构成。

　　Hold yourself for a moment while I take your photograph.不要动，我给你照张像，

　　Hold yourself upright when you walk.走路时身体要挺直。

　　Would you please hold the door open for me?劳驾你给我把门拉开好吗?

　　We’ve held the job open for him for two weeks now.我们已将这个工作位置为他保留两周了。

　　▲ in place是固定词组，意为“在原来的地方”。如：

　　I like everything to be in place.我喜欢一切东西都放在原来的地方。

　　I hope you leave all the books in the library in place.。我希望你把所有的书放在图书馆原来的地方。

　　The picture is not in place on the wall. 这幅画不在墙上原来的地方

　　I wish you would keep things in their places. Where is my hat? 但愿你能把东西放在原来的地方，我的帽子呢?

　　7. So when it rains, soil is washed down the hills and into the rivers. Strong winds can also blow away the valuable soil that is on the top of the f