【知识点详解】
1. 感叹句构造：在英语中，当我们要表达对某事物的惊叹或感叹时，可以使用感叹句。本题中，将陈述句"The mountain is very beautiful."改为感叹句，通常有两种形式："What + (a/an) + 形容词 + 名词 + 主语 + 动词！" 或 "How + 形容词/副词 + 主语 + 动词！"。所以原句改为"What a beautiful mountain it is!"。
2. 同义句转换：英语中，询问天气的句型"如何"可以用"How is the weather?"或"What is the weather like?"，所以"How is the weather in China?"等同于"What's the weather like in China?"。
3. 比较级与最高级："The boy is the most careless in his class."表示男孩是班里最粗心的。要改为同义句，可以使用比较级，比如"The boy is more careless than any other boy in his class."，这表示他比班上的其他任何男孩都粗心。
4. "Neither...nor..."结构：原句"He didn't watch TV, his mother didn't either."表示他没看电视，他妈妈也没看。使用"Neither...nor..."可以表达相同的意思："Neither he nor his mother watched TV."
5. "spend + 时间 + (in) doing sth."结构：原句"I spent the whole afternoon learning English."表示我整个下午都在学英语。这个句型也可以转换为"I spent all the afternoon on learning English."或"I spent the whole afternoon in learning English."
6. "spend + 钱 + on sth."、"pay + 钱 + for sth."、"sth. cost sb. 钱"的用法：原句"I spent 100 yuan on the coat."可以转换为"I paid 100 yuan for the coat."、"I spent 100 yuan buying the coat."以及"The coat cost me 100 yuan."
7. 对时间长度提问：句子"He has worked in this school since 2 years ago."中，since引导的时间状语表示一段时间，对此提问使用"How long"。
8. "It is/has been + 一段时间 + since..."结构：原句"He joined the army 3 months ago."可以转换为"It is/has been 3 months since he joined the army."，表示他加入军队已经三个月了。
9. 同义句转换："have a good time"与"enjoy oneself"都有“玩得开心”的意思，所以"The visitors had a good time in Beijing."等同于"The visitors enjoyed themselves in Beijing."
10. 对将来时间提问：对"in + 一段时间"提问，使用"How soon"，故"He will come back in a week."提问为"How soon will he come back?"
11. 反意疑问句：对于否定陈述句的反意疑问句，通常使用肯定形式。"He has never been abroad."的反意疑问句是"He has never been abroad, has he?"
12. "Don't"开头的祈使句反意疑问句：祈使句的反意疑问句通常使用"will you"，所以"Don't go alone, will you?"。
13. "There be"句型的反意疑问句：对于"There be"句型，若陈述部分含否定词"little"，反意疑问句使用肯定形式。所以"There is little meat in the fridge, isn't there?"
14. 提建议的句型："Why not + 动词原形？"表示提建议，所以"Why not ask the man over there?"。
15. 询问人口数量：用"What's the population of + 地名？"来询问某地的人口数量，因此"How many people are there in India?"可以转换为"What's the population of India?"。
以上是对题目中涉及的各个知识点的详细解释和拓展。
