3. In other words, there are three times as many girls as boys.
也就是说，女生人数是男生的三倍。
表示A是B的几倍时，常用以下句式：
(1)A is x times as＋形容词(或副词)原级＋as B
There is x times as many＋可数名词复数＋as B
There is x times as much＋不可数名词＋as B
(2)A is x times＋形容词(或副词)比较级＋than B
(3)A is x times＋the size / height / length / width＋of B

4. Oh really? So have I. Which language are you studying?
哦,是吗？我也是。你在学什么语言？
“so +助动词/情态动词/系动词+主语”表示“(另一事物)也……”
He is sixteen, so am I.
他16了，我也是。
Tom likes playing football, so do I.
汤姆喜欢踢足球,我也是。
(1)表示否定意义时用“neither/nor +助动词/情态动词/系动词+主语”，意为“(另一事物)也不……”。
He doesn't like math, neither do I.
他不喜欢数学,我也不喜欢(数学)。
I don't know about it, nor/neither do I care.
我不知道这件事,也不关心。
(2)“so +主语+助动词/情态动词/系动词”表示对之前或对方所说的情况表示赞同或证实，意为“(同意一个人或事物)确实……”
— Mary is good at music.
——玛丽音乐很好。
— So she is.
——是的，的确很好。
— They have moved away.
——他们已搬走了。
— So they have.
——是的，他们已搬走了。
注意：助动词及其时态的选择要根据前句确定。
He has finished his homework, so has Mary.(so have the other students.)
他已经完成了他的家庭作业,玛丽也完成了。(其他学生也完成了。)
The teacher will go to America, so will the students.
老师要去美国,学生们也要去。
（3）So it is / was with me. 我也如此。用于前面句子中并列谓语不同或肯、否定形式不同时。

5. The school year is divided into two semesters, the first of which is September through December, and the second January through May.
一学年有两个学期,第一学期从九月到十二月,第二学期从一月到五月。
本句中含有一个“介词+关系代词”构成的非限制性定语从句。“介词+关系代词”引导定语从句时，关系代词只能用which、whom，不能用that，也不能省略。
We live in a house, in front of which is a small river.
我们住在前面有一条小河的房子里。