是以重读闭音节结尾，末尾又只有一个辅音字母时，将末尾的辅音字母双写再加-ed。
例如
put、drag
例外：
1、以r结尾，末尾音节又重读的r要双写，如prefer-preferred.如r前为双元音时，r则不需双写。如，appear-appeared;
个别以r结尾的单音节要双写：star-starred;bar-barred.
2、以l结尾，l前为长音，则l一概不双写，如：reveal-revealed.
3、以x结尾的词直接加ed.
补充：什么是闭音节?
以一个或几个辅音结尾(除r外)，而且中间只有一个元音的音节，我们称之，闭音节。
buy的过去式和过去分词都是bought。buy作名词时意思是“购买，买卖，作及物动词时意思是“购买、获得、贿赂、相信”，作不及物动词时意思是“买，采购”。buy的复数是buys， 第三人称单数是buys， 现在分词是buying 。
1、buy用过去式和过去分词时，例句有：
Lizzie bought herself a mountain bike. 莉齐给自己买了一辆山地自行车。
I bought some new clothes for the trip. 我为这次旅行买了一些新衣服。
As an experiment, we bought Ted a watch. 作为尝试，我们给特德买了块手表。
The house was a wreck when we bought it. 我们买下这座房子时，它破烂不堪。
I've just bought a new house. 我刚买了一个新房子。
Any number of our military and government officials can be bought.
我们的军官和政府官员中有许多可以被收买。
2、buy用原形作及物动词时，例句有：
He could not afford to buy a house.他买不起房子。
It was a risky operation, but might buy more time.
这是一项冒险的举动，但也许能换得更多的时间。
3、buy用复数时，例句有：
About $70,000 buys a habitable house.
大约七万美元才买得到一栋适合居住的房子。
4、buy用现在分词形式时，例句有：
We were warned off buying the house. 有人劝我们不要购买这所房子。
We're buying a new TV with the money. 我们要用这笔钱买一台新电视机。
5、buy的短语有：We Bought a Zoo 我家买了动物园 ; bought note 买货单 ;
bought and sold note 买卖单据 ;foreign exchange bought 买入外汇 ; Be
bought a lesson 吃一堑长一智
6、buy的同根词有：buyer 买主;采购员
Diana is a buyer for a chain of furniture shops.
戴安娜是一家连锁家具商店的采购员。
Car buyers are more interested in safety and reliability than speed.
比起速度，汽车买主更关注安全性和可靠性。
短语有：BUYER READING 买家须知 ; Retail Buyer 零售采购员 ; Wholesale
Buyer 批发采购员
7、buy的反义词是：sell出卖、销售
Catlin sold the paintings to Philadelphia industrialist Joseph Harrison.
卡特林把这些画卖给了费城实业家约瑟夫·哈里森。
The directors sold the business for $14.8 million.
董事们把这个公司以1480万美元卖掉了。
短语有：sell out 卖完 ; 售完 ; sell off 变现 ;Sell short 卖空
