网站的类型及特点seo工程师是做什么的
文章目录
- 疑问
- 解释
- 1. 表达过去的推测
- 2. 与时态一致
- 3. 语法结构的限制
- 4. 例子对比
- 总结
疑问
This must have been a year-round activity as no structures have been found which would have been used to shelter animals in the winter.
为什么must表示对过去推测要用完成时，不能直接用过去时吗？
解释
使用 “must have been” 而不是简单的过去时，有以下几个原因：
1. 表达过去的推测
“must have been” 是一种用于表达对过去情况的推测或推断的结构。它由情态动词 “must” 加上 “have” 和过去分词 “been” 组成，用来表示说话人基于现有证据，对过去某个时间点或期间的情况进行高度肯定的推测。
例句：
- 正确：“This must have been a year-round activity.”
- 意思是：“这一定是全年进行的活动。”
- 错误：“This must was a year-round activity.”
- “must was” 这种搭配在语法上是不正确的。
2. 与时态一致
在你的句子中，讨论的是过去的活动和过去的证据（“no structures have been found”）。因此，使用 “must have been” 保持了时态的一致性，明确表示这是对过去情况的推测。
-
时态一致：
- “must have been”（对过去的推测）与 “have been found”（现在完成时，表示过去到现在的状态）相匹配。
-
如果使用简单过去时：
- 英语中，情态动词 “must” 不与简单过去时直接搭配。要表达过去的推测，必须使用 “must have + 过去分词”。
3. 语法结构的限制
情态动词 “must” 后面不能直接跟过去式动词。如果需要表达对过去的推测，必须使用 “must have” 加上过去分词。
-
正确：
- “must have been”
- “must have done”
-
错误：
- “must was”
- “must did”
4. 例子对比
-
对现在的推测：
- “She must be at home now.”
- 意思是：“她现在一定在家。”
- “She must be at home now.”
-
对过去的推测：
- “She must have been at home yesterday.”
- 意思是：“她昨天一定在家。”
- “She must have been at home yesterday.”
总结
在你的句子中：
“This must have been a year-round activity as no structures have been found which would have been used to shelter animals in the winter.”
使用 “must have been” 是正确的，因为你在基于过去的证据对过去的活动进行推测。简单的过去时无法正确表达这种推测的含义和时态。
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