2) It's well-known that Americans are competitive...众所周知，美国人喜欢争强好胜。

3) For an American girl, it's so important that you have a good time at the prom.对于美国女孩来说，在舞会上玩得快活太重要了。

4) It's a great pity that it's probably the last time this will happen.真遗憾，这样美妙的夜晚很可能不会再有了。

但是4)后半部分...it's probably the last time this will happen中的it不是形式主语。

3. Something else I've greatly enjoyed is working as a peer mediator, someone who helps students to settle problems that they have with other students. 另外一件我非常乐意做的事情是做同伴调解人，就是帮助学生们解决同学之间的问题。

work as 以某一种身份做工作，例如：

Those days, he worked as an English teacher in a village school in China.

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have problems with...和......之间有问题，出现了麻烦，例如：

What problems do you really have with him?

How many low-income immigrants and their families have problems with access to mental health treatment because of a language barrier and a deep cultural divide.

4. At the start of the school year, we were given training in how to do this...新学期开始，我们就接受了培训，学习怎样做这种工作。

how to do this为带疑问词的不定式，做介词in的宾语。有时要在不定式前加一个疑问词作动词或介词的宾语，例如

He often teaches us how to pronounce English words.

Have you got an idea of where to get help?

5....and I did enjoy it whenever I won our races! 不管什么时候只要我赢了比赛，我都会为此欣喜万分！

　do或did放在动词原形前，加强该动词的语气。例如：

I do think we should start as soon as possible.

He did tell me he would leave for London.

Do come to China next time.

Do come to my birthday party. 一定来参加我的生日宴会。

I did go there. 我确实去那儿了。

I do miss you. 我确实想你。

句中的it是人称代词，代表抽象的事物，指的是whenever I won our races。

whenever引导时间状语从句，意思为every or any time，例如：

You may use my dictionary whenever you want to.

He was always in his lab whenever I went to see him.

6. It took two hours but it was worth it, as everyone told me I looked very elegant! 这花了我两小时，不过很值得，因为每个人都说我看起来端庄典雅。

在it took two hours but it was worth it.这句中

第一个"it"指的是"做头发这件事" -- to have my hair done

第二个"it"指的也是"做头发这件事" --to have my hair done

第三个"it"指的是"两小时的时间" --the time of two hours, 在这里，可理解为：worth it=worthwhile在上下文意思明确时，我们可以说worth it，表示一个很好的理由去做某事、值得去做某事。这里的it代表什么，可以通过上下文去理解，例如：

It was hard work, but it was worth it in the end.