1
In one case, [clause]
用于句首，引出某个具体情况。
In one case, the patient recovered completely.
在一次案例中，病人完全康复了。
有一次
先理解核心含义，再看它在真实句子里的用法。
有一次
on one occasion
In one case, the patient recovered completely.
在一次案例中，病人完全康复了。
相关中文义项：有一次
on one occasion
In one case, the package was delivered to the wrong address.
在一个案例中，包裹被送到了错误的地址。
She skipped class in one case, but otherwise she was a diligent student.
她有一次逃课，但除此之外她是一名勤奋的学生。
In one case, the experiment produced unexpected results.
在一次实验中，产生了意想不到的结果。
用于句首，引出某个具体情况。
In one case, the patient recovered completely.
在一次案例中，病人完全康复了。
用于句末，强调某一特定情况。
The experiment failed in one case.
实验在一次案例中失败了。
用“only”强调唯一性，常引起倒装。
In only one case did the treatment show no effect.
仅在一种情况下，治疗未显示效果。
Not this
In a case, I forgot my keys.
Use this
In one case, I forgot my keys.
使用 'a' 代替 'one'，导致泛指与特指混淆。'in one case' 特指某一次具体情况，而 'in a case' 通常指在某个案例或情况下，不符合此处 '有一次' 的含义。
Not this
In case, he arrived late.
Use this
In one case, he arrived late.
混淆了 'in case'（以防）与 'in one case'（有一次）。'in case' 是连词引导目的状语，而此处需要表示 '在某一次情况中'，因此必须用 'in one case'。
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Focus on the distinctions below. Other candidates are kept compact until a reliable usage note is available.
Use in one case naturally, then get feedback on meaning, tone, grammar, and phrasing.
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常用于举例或引出某一具体情形，可置于句首或句末。
Dictionary facts from ECDICT, WORDNET.