she's的问题
She's cast off three boy-friends in a month.
这里she's = she has，
has/have + 过去分词，是现在完成时态的结构。句意是：她一个月里已经抛弃了3个男友。
she's 还可以是she is的缩写，但是上句不是，如果是is，句子需要进行变形：She's cast off 【by】 three boy-friends 【去掉in a month】.或者：She has been cast off by three boy-friends in a month.
解释：be + 过去分词，这是被动语态的结构，要由介词by引出动作的执行者，所以上句要加上介词by。
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istres能组成什么单词
istres组成的单词是sister.sister单词发音：英 [ˈsɪstə] 美 [ˈsɪstər]。一、sister单词释义n. 姐妹；修女；护士v. 姐妹般对待adj. 姐妹般的；同类型的二、词形变化复数: sisters三、短语搭配sister organization 姊妹团体四、双语例句1、My best friend has been like a sister to me. 我最要好的一个朋友待我亲如姐妹。2、John had no brothers or sisters and wasn't used to sharing. 约翰没有兄弟姐妹，所以不习惯和他人分享东西。3、She doesn't play as well as her sister. 她演奏得不如她姐姐。4、My sister and I used to quarrel all the time. 我和妹妹过去老是吵架。5、My sister came along just to give me some moral support. 我姐姐只是过来给我一些道义上的支持。6、She's never really got on with her sister. 她从未与妹妹真正和睦相处过。7、I'm going around to my sister's later. 我打算不久后到姐姐家去看看。8、We have our differences, but she's still my sister. 我们之间虽然不和，但她仍旧是我的妹妹。9、Her sister is a big girl, isn't she? 她姐姐是个大个子胖姑娘，是吗？10、She's even more intelligent than her sister. 她甚至比她姐姐还聪明。11、She was far more intelligent than her sister. 她比她姐姐聪明多了。12、I slept at my sister's house last night. 昨晚我住在妹妹家了。