twin是双胞胎的意思对吧,可它是指一个还是一对,什么时候该加s?加和不加的时候谓语动词分别用什么形式?

twin指双胞胎中的一个人,谓语动词用第三人称单数；加了twins指双胞胎两个人,用第三人称复数的谓语形式
not only but also
用法主要是根据“就近原则”
详细的请参考以前知道里别人曾经回答过的,我认为很好
（以下为详细用法）
“not only … but also …”是一个并列连词词组,其意思基本等于“both … and …”,但侧重点放在“but also”上,另外该词组使用时须遵守一定的规则,如要求对称,倒装及主谓一致等.以下两个句子往往被看作是欠妥或错误的：
1.*Sitting up late last night,Tom not only read the assignment but also many poems by his favourite poet.
2.*Not only the students but also the teacher were invited.
上述两句第一句欠妥,第二句是错误的,应分别改为：1.Sitting up late last night,Tom read not only the assignment but also many poems by his favourite poet.昨晚汤姆熬至深夜,不仅看了课外作业,而且读了他最喜欢的一位诗人的许多诗歌.2.Not only the students but also the teacher was invited.不仅学生们被邀请,而且那位教师也被邀请了.下面就来谈谈正确使用该词组时须注意的几点事项及其常见的几种变体形式.
一、使用not only … but also … 时须注意的几点：
1.not only与but also后面所连接的词的词性必须对等：
(1) Frankin was considered not only an inventor,but also a statesman.富兰克林不仅被看作发明家,而且被看作政治家.
(2) The nurse was not only competent but also kind.这位护士不仅能干而且亲切和蔼.(3) They not only broke into his office and stole his books,but also tore up his manuscripts.他们不仅闯进他的办公室,偷走了他的书,而且还撕掉了他的手稿.
(4) Not only you but also she has to attend the ceremony.不令你而且她也得参加典礼.
(5) In production,we should always keep an eye not only on quantity but also on quality.在生产中,我们不仅要关注数量,而且要关注质量.
(6) They completed the project not only punctually but also perfectly.他们不仅准时完成工程,而且完成得很出色.2.not only只能连用,而but also既可连用,也可分开用：a.He speaks not only English,but also French.他不仅说英语,还说法语.b.Television is not only boring,but it also wastes a lot of time.电视不仅乏味,而且还浪费许多时间.c.She was not only compelled to stay at home,but she was also forbidden to see her friends.她不仅被强迫蹲在家中,而且被禁止去看朋友.3.谓语动词的数应与but also后主语的数保持一致：Not only you but also Mr Zhang teaches in this college.不仅你,张老师也在此学院教书.
4.not only放在句首,后接句子时要用倒装结构：
(1) Not only should proletarians emancipate themselves but also the whole mankind.无产者不仅要解放他们自己,而且要解放全人类.
(2) Not only does television appeal to those who can read but to those who can't.电视不仅吸引阅读的人,而且也吸引了不会阅读的人.
(3) Not only was everything Albert Einstain had taken away but also his citizenship was deprived of.爱因斯坦的财产不仅被掳掠一空,而且他的德国国籍也被剥夺了.
(4) Not only had the poor man been fined,but also he had been sent to prison.这个可怜的人不仅被罚款,而且还被送进了监狱.二、not only,but also的几种常见的变体形式：
1.not only的变体形式：常见的变体有not just,not merely,not simply,not solely等.
(1) Such work is not just devalued in that country,its nature is widely misunderstood.这种工作在那个国家不但被贬低,而且工作的性质也被许多人误解.
(2) There is not merely concision in these lines but also elegance.这些台词写得不仅乘法,而且优雅.
(3) Not simply did he teach school,but he wrote novels.他不仅教书,也写小说.
(4) This novel is not solely suitable for us in studying modern fiction but can also be used in the teaching.这本小说不但适合我们研究现代小说,而且能用于教学.
2.but also的变体形式：a:省去also,只留but；b:省去but,只留also；c:but…as well；d:but also全部省略.
(1) Scarlett quaked lest she and Frank lose not only their freedom but the house,the store and mill.斯佳丽害怕得发抖,惟恐她与弗兰克不仅会失去自由,而且还会失去房屋、商店及锯木厂.
(2) She's not only an excellent housewife also a first class mathematician.她不仅是一个出色的家庭主妇,而且是一位一流的数学家.
(3) Not only does he write the words to the songs,but he composes the music as well.他不仅给歌曲写词,也谱曲.
(4) Such a change would improve not only his social image but his health as well.这样的变化不仅会提高他的社会形象,而且会改善他的健康.
(5) Not only did I hear the car,I actually saw it crash.我不仅听见了车的声音,而且还亲眼看见了那辆车撞坏了.
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他不但自信,而且很安静
He is not only confident but also quiet.

1、单数表示“双胞胎之一”。如：He is the younger twin of the two. 加“s”指两个人，就是双胞胎。 She gave birth to twins. 她生了双胞胎。
“双胞胎之一”可以用“one of the twins”
2、not only...but also...
A、正常用法，几乎后面可以加任何词性的词。看例句：
(例句都...

twin不加s的话就只是指双胞胎之一而已。 当说的是双胞胎两个人的话就要加s 了，加了s谓语当然就是复数咯。
当not only放在句首的时候才需要倒装，看看下面的例子吧：
He is not only strong and healthy， but also very smart.
他不仅强壮健康，而且还很聪明。
Not only did the dog bark...