Farmers need water to water the crops and vegetables.

Farmers need water to water the crops and vegetables.
为什么在vegetables前就不用加the了呢?
Leaves are green还是The leaves are green?
到底在什么情况下要加The?什么情况下不加?

and连接crops和vegetables,当然省去the,leaves为复数可数名词,复数时不特指不加the
used to refer to somebody/ something that has already been mentioned or is easily understood（指已提到或易领会到的人或事物）
There were three questions.The first two were relatively easy but the third one was hard.有三个问题.头两个相对容易,第三个困难.
There was an accident here yesterday.A car hit a tree and the driver was killed.昨天这里发生了一起事故.一辆小轿车撞到树上,驾车的人死了.
The heat was getting to be too much for me.天气热得快让我受不了了.
The nights are getting longer.夜越来越长.
used to refer to somebody/ something that is the only,normal or obvious one of their kind（指独一无二的、正常的或不言而喻的人或事物）
the Mona Lisa 《蒙娜莉萨》
the Nile 尼罗河
the Queen 女王
What's the matter?怎么回事?
The phone rang.电话铃响了.
I patted her on the back.我拍了拍她的背.
How's the (= your)baby?宝宝好吗?
used when explaining which person or thing you mean（解说时用）
the house at the end of the street 街尽头的房子
The people I met there were very friendly.我在那里遇到的人很友善.
It was the best day of my life.这是我一生中最美好的一天.
You're the third person to ask me that.你是第三个问我那件事的人.
Friday the thirteenth 十三号,星期五
Alexander the Great 亚历山大大帝
used to refer to a thing in general rather than a particular example（用以泛指）
He taught himself to play the violin.他自学拉小提琴.
The dolphin is an intelligent animal.海豚是聪明的动物.
They placed the African elephant on their endangered list.他们把非洲大象列为濒危动物.
I heard it on the radio.我从收音机里听到了这件事.
I'm usually out during the day.白天我通常不在家.
used with adjectives to refer to a thing or a group of people described by the adjective（与形容词连用,指事物或统称的人）
With him,you should always expect the unexpected.在他身上你应随时料到有意想不到的事情发生.
the unemployed 失业者
the French 法国人
used before the plural of somebody's last name to refer to a whole family or a married couple（用于姓氏的复数形式前,指家庭或夫妇）
Don't forget to invite the Jordans.别忘了邀请乔丹一家.
enough of something for a particular purpose（指特定用途的事物）足够,恰好
I wanted it but I didn't have the money.我想买那东西,但钱不够.
used with a unit of measurement to mean 'every'（与计量单位连用）每,一
My car does forty miles to the gallon.我的车每加仑汽油跑四十英里.
You get paid by the hour.你领的是时薪.
used with a unit of time to mean 'the present'（与时间单位连用）当前的,本,此
Why not have the dish of the day?为什么不吃今天的精选菜?
She's flavour of the month with him.她是他眼下的红人.
/ði:/used,stressing the,to show that the person or thing referred to is famous or important（重读,表示所指的为知名或重要的人或事物）
Sheryl Crow?Not 'the Sheryl Crow?谢里尔 克罗?莫不是大名鼎鼎的谢里尔 克罗?
At that time London was 'the place to be.那时候伦敦已具备了大城市的基础.
IDMthe more,less,etc.,the more,less,etc.used to show that two things change to the same degree（用以表示两个事物按照同一程度变化）越…越,愈…愈
The more she thought about it,the more depressed she became.她越想这事越沮丧.
The less said about the whole thing,the happier I'll be.对整个事情议论得越少,我越高兴.