语法点：非真实条件句：与现在事实相反 (Second Conditional)
第二条件句（非真实条件句）用来谈论与现在事实相反或不太可能实现的假设：if 从句用一般过去时，主句用 would + 动词原形。它并不表示过去，而是表示“假如……就会……”的想象。本课讲述海底隧道的几个方案，工程师在提出设想时大量使用 would / if… were…，因为这些都是当时尚未实现的假设。课文第 9 句还出现了第三条件句（与过去事实相反：if + had done, would have done），可作对比。
结构
| 类型 | if 从句 | 主句 | 含义 |
|---|---|---|---|
| 第二条件句 | if + 一般过去时 | would / could / might + 动词原形 | 与现在事实相反或不太可能 |
| 第三条件句 | if + had + 过去分词 | would have + 过去分词 | 与过去事实相反 |
- 否定：If it did not rain, the fire would not go out.
- 疑问：What would you do if you had a million pounds?
- if 从句中的 be 动词常用 were（If I were you…）。
- 叙述过去时的设想（“过去将来”）也用 would：He said that it would be possible.
课文中的例句
- [句 4] The tunnel would be well-ventilated if tall chimneys were built above sea level. — 典型第二条件句：if + were built，主句 would be。
- [句 7] This would solve the problem of ventilation for if a train entered this tunnel, it would draw in fresh air behind it. — if + entered（过去式），主句 would draw，表示设想中的情况。
- [句 2] He said that it would be possible to build a platform in the centre of the Channel. — would 表示设想中的可能，与 said 一起构成过去将来。
- [句 3] This platform would serve as a port and a railway station. — 主句单独出现的 would，条件隐含在上文的计划中。
- [句 9] If, at the time, the British had not feared invasion, it would have been completed. — 对比：第三条件句，与过去事实相反。
补充例句
- If the tunnel were built, trains would run under the sea. — 如果建成隧道，火车就能在海底行驶。
- If I had a plan like that, I would show it to the engineers. — 如果我有那样的计划，我会拿给工程师们看。
- The platform would not be safe if the sea were rough. — 如果海上风浪大，这个平台就不安全。
易错点
- ❌ The tunnel would be well-ventilated if tall chimneys are built above sea level. → ✅ …if tall chimneys were built above sea level. —— 与现在事实相反的假设，if 从句要用过去式，不用现在时。
- ❌ If a train entered this tunnel, it will draw in fresh air. → ✅ If a train entered this tunnel, it would draw in fresh air. —— 主句要用 would，不用 will。
- ❌ If a train would enter this tunnel, it would draw in fresh air. → ✅ If a train entered this tunnel, … —— if 从句里不用 would。
- ❌ If the British had not feared invasion, it would be completed. → ✅ …it would have been completed. —— 与过去事实相反要用 would have + 过去分词。