Economic development is necessary if we want to improve society. There comes a time when the old must give way to the new, and it is not possible to preserve everything from our past as we move towards the future. Finding and keeping the right balance between progress and the protection of cultural sites can be a big challenge.
There comes a time when the old must give way to the new 中 when 引导定语从句修饰 a time，表示"到了……的时候"。
类似结构：There comes a point when you have to make a decision. 到了你必须做决定的时候了。
Finding and keeping... 动名词短语作主语
Finding and keeping the right balance 是动名词短语作主语，谓语用单数 can be a big challenge。
并列动名词由 and 连接时，如果表示一个整体概念，谓语用单数。此处"寻找并保持平衡"被视为一个整体过程。
Big challenges, however, can sometimes lead to great solutions. In the 1950s, the Egyptian government wanted to build a new dam across the Nile in order to control floods, produce electricity, and supply water to more farmers in the area. But the proposal led to protests. Water from the dam would likely damage a number of temples and destroy cultural relics that were an important part of Egypt's cultural heritage. After listening to the scientists who had studied the problem, and citizens who lived near the dam, the government turned to the United Nations for help in 1959.
would likely damage 中 would 表示推测（"很可能"），likely 是副词修饰 would damage，增强推测语气。
区别：likely 作形容词时常用于 be likely to do sth 结构。此处 likely 是副词，相当于 probably。
After listening to... 介词+动名词作状语
After listening to the scientists who had studied the problem, and citizens who lived near the dam 中 After 是介词，后接动名词 listening 作时间状语。两个 who 引导的定语从句分别修饰 scientists 和 citizens。
这种结构比 After the government listened to... 更简洁，主语需与主句一致（政府）。
A committee was established to limit damage to the Egyptian buildings and prevent the loss of cultural relics. The group asked for contributions from different departments and raised funds within the international community. Experts investigated the issue, conducted several tests, and then made a proposal for how the buildings could be saved. Finally, a document was signed, and the work began in 1960.
The project brought together governments and environmentalists from around the world. Temples and other cultural sites were taken down piece by piece, and then moved and put back together again in a place where they were safe from the water. In 1961, German engineers moved the first temple. Over the next 20 years, thousands of engineers and workers rescued 22 temples and countless cultural relics. Fifty countries donated nearly $80 million to the project.
a place where they were safe from the water 中 where 引导定语从句修饰 a place，where = in which。
先行词为地点名词时，定语从句可用 where/in which 引导，但要注意从句是否缺主/宾语：缺则用 which/that，不缺则用 where。
When the project ended in 1980, it was considered a great success. Not only had the countries found a path to the future that did not run over the relics of the past, but they had also learnt that it was possible for countries to work together to build a better tomorrow.
Not only had the countries found 使用了部分倒装。当 Not only 置于句首时，其所在的分句需部分倒装（将助动词 had 提至主语前），but also 分句不倒装。
Not only did he finish the task, but he also did it well. 他不仅完成了任务，而且做得很好。
正常语序：The countries had not only found...but had also learnt...
it was possible for...to... 不定式复合结构
it was possible for countries to work together 中 it 是形式主语，真正主语是 for countries to work together。
for sb to do sth 是不定式复合结构，for 引出不定式的逻辑主语。这种结构比 that countries could work together 更简洁。
The spirit of the Aswan Dam project is still alive today. Perhaps the best example is shown by UNESCO, which runs a programme that prevents world cultural heritage sites around the world from disappearing. If a problem seems too difficult for a single nation, the global community can sometimes provide a solution.
UNESCO, which runs a programme that prevents world cultural heritage sites...from disappearing 包含两个定语从句：which 引导非限制性定语从句修饰 UNESCO；that 引导限制性定语从句修饰 a programme。
UNESCO（联合国教科文组织）：United Nations Educational, Scientific and Cultural Organization 的缩写，1945年成立，总部在巴黎，旨在通过教育、科学、文化促进国际合作，维护世界和平与安全。其世界遗产名录（World Heritage List）已收录超过1100处遗产。