倒装句是英语语法中的一种特殊结构，用于强调句子中的某些成分。助动词的选择在倒装句中非常重要，包括do/does/did, has/have/had, will/would, can/could等，它们在不同的情况下表达不同的意义。例如，do/does/did用于主语为第三人称单数的情况，has/have/had用于现在完成时或过去完成时，will/would用于将来时或虚拟语气，can/could用于表达能力或虚拟语气。
倒装句的构成
倒装句是指将谓语动词的一部分提到主语之前，构成的特殊语序。一般来说，倒装句分为完全倒装和部分倒装两种形式。例如：
- 完全倒装：Not only did he finish his homework, but he also cleaned the room.
- 部分倒装：She never goes to bed before midnight. -> Never does she go to bed before midnight.
助动词的选择
在倒装句中，助动词的选择非常重要，不同的助动词可以表达不同的意义。常见的助动词有do/does/did, has/have/had, will/would, can/could等。下面我们来看一些例子。
Do/does/did
当主语为第三人称单数时，倒装句中需要用到do/does/did来构成。例如：
- Never does she eat meat. (她从不吃肉。)
- Rarely did he complain about the food. (他很少抱怨食物。)
Has/have/had
当主语为现在完成时或过去完成时，倒装句中需要用到has/have/had来构成。例如：
- Only once has she been to Paris. (她只去过一次巴黎。)
- Never had I seen such a beautiful sunset before. (我从未见过如此美丽的日落。)
Will/would
当主语为将来时或虚拟语气时，倒装句中需要用到will/would来构成。例如：
- Not until next week will we have a meeting. (直到下周我们才会开会。)
- Were it not for your help, I would have failed the exam. (如果不是你的帮助，我就会考砸了。)
Can/could
当主语为能力时或虚拟语气时，倒装句中需要用到can/could来构成。例如：
- Only when you practice regularly can you improve your English. (只有当你经常练习，你的英语才会提高。)
- Had I known about the traffic, I could have left earlier. (如果我知道交通情况，我本来可以早点离开的。)
