另外，be sure + 不定式大多数情况下也用于祈使句，表示“务必；一定要；不要忘记”。
例如：
Be sure to come to our party if you have time.
如果有时间的话请一定来。
Be sure to finish the work as soon as possible.
请务必尽快完成这项工作。
6. Tie the tree to the top of the stick to keep it straight.
把树系在棍子的顶部让他直着。
(1) tie v.“系上”；n.“领带，领结”；tie…to…意为“把……系在……”。
She tied his horse to a tree by the road.
他把马拴在路旁的一棵树上。
She bought a tie for her husband yesterday.
她昨天给她丈夫买了条领带。
(2) to keep it straight 是动词不定式，在句中作状语，表示捆树的目的。keep it straight 为“动+ 宾 + 宾补”结构，如：
You'd better keep your room clean. 你最好使你的房间保持清洁。
Don't touch the machine. You must keep yourselves away from any danger. 别动机器，你该保证自己的安全。
7. But more “Great Green Walls” are still needed, and not only in China.
但是，更多的绿色长城仍旧需要，这不仅在中国。
not only 能常和but also连用，意思是“不但……而且……”，可以连接两个主语、谓语、宾语等，当连接两个主语时，谓语动词应和第二个主语在人称和数上保持一致，also往往可以省略。例如：
She is good at not only dancing but (also) singing. She is both a dancer and a singer.
她不但善长跳舞，而且善长唱歌。（连接两个宾语）她是个舞蹈家和歌唱家。
Not only you but (also) your sister has to get to school on time. Being on time is a must.
不仅你而且你妹妹都得按时到校。守时是必须的。
8. Many thousands of trees must be planted every year.
每年必须种植成千上万棵树。
（1）thousands of这种类型的结构前面已讲述过，它的前面不可用具体的数字，可用不确定数目的修饰语，如some, many, several等。
Some hundreds of people watched the match.
好几百人观看比赛。
（2）plant / grow
这两个动词都有“种”的意思，区别如下：
1) plant“种植”，及物或不及物均可，常指移植已长成秧苗的植物，可用于“plant+场地+with”结构。如：
They planted trees in the garden. = They planted the garden with trees. 他们在花园里种树。
April is time to plant. 四月是种植季节。
2）grow“栽培；（人或草本等）生长”，及物或不及物均可，常指使某种植物从种子起在某地生长，不移走。如：
They grow roses in the greenhouse. 他们在温室里栽培玫瑰花。
Do all plants grow from seeds? 所有的植物都从种子长成的吗？
3) 指种植花草，用grow或plant均可；指种植树木，一般用plant；指种植农作物，用grow。如：
We have grown / planted a lot of flowers this summer. 我们夏天栽培了许多花。
People in the south grow rice. 南方人种植水稻。
9. The more, The better.
越多越好。
So the more trees there are, the better harvests we have.
因此树木越多，我们的收成也越好。
the +比较级 + …，the + 比较级 +…。这个句型的意思是“越……，越……”。例如：
The busier the farmers are, the happier they feel. The more students read, the better their English will be. The longer plant grows, the better.
农民们越忙越高兴。学生读得越多，他们的英语会越好。庄稼的生长期越长，长得越好。
10. In a few years’ time, those mountains will be covered with trees, too.
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