虚拟语气¶
虚拟语气表示与事实相反、不太可能实现的假设或愿望。它的标志是 时态往过去退一步：说现在的事用过去时，说过去的事用过去完成时。
if 条件句¶
先区分真实条件和虚拟条件：
- If it rains tomorrow, I will stay home.（真实条件：明天有可能下雨）
- If I were you, I would go.（虚拟条件：我不可能是你）
虚拟条件句按假设的时间分为三种：
| 假设的时间 | if 从句 | 主句 | 示例 |
|---|---|---|---|
| 与现在事实相反 | 过去式（be 一律用 were） | would / could / might + 动词原形 | If I were rich, I would travel the world.（我现在并不富有） |
| 与过去事实相反 | had + 过去分词 | would / could / might + have + 过去分词 | If I had studied harder, I would have passed the exam.（我当时没好好学） |
| 与将来事实可能相反 | 过去式 / should + 动词原形 / were to + 动词原形 | would / could / might + 动词原形 | If it should rain tomorrow, we would cancel the trip.（明天不太可能下雨） |
if 从句中含有 were、had、should 时，可以省略 if，把它们提到句首：Had I known, I would have told you. 见特殊句式 · 部分倒装。
wish¶
wish 后面的从句表示难以实现的愿望：
| 愿望的时间 | 从句 | 示例 |
|---|---|---|
| 现在 | 过去式（be 用 were） | I wish I were taller. |
| 过去 | had + 过去分词 | I wish I had listened to you. |
| 将来 | would / could + 动词原形 | I wish it would stop raining. |
其他常见结构¶
| 结构 | 从句形式 | 示例 |
|---|---|---|
| as if / as though | 与现在事实相反用过去式，与过去事实相反用 had done | He talks as if he knew everything. |
| It's (high) time that … | 过去式 | It's time we went home. |
| would rather + 从句 | 过去式 | I'd rather you stayed here. |
| suggest、insist、demand、order、advise、require + that 从句 | (should) + 动词原形 | He suggested that we (should) leave early. |
| It is important / necessary / essential that … | (should) + 动词原形 | It is important that he (should) be on time. |
最后两种结构中，should 可以省略，但动词仍用原形：He suggested that she go（不是 goes）。