When does "أَنْ" mean "lest / so that not"?

Yes, exactly — the Arabic particle "أَنْ" may indeed be used to express the meaning of:

✅ “so that not” or “lest”

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🧠 When does "أَنْ" mean "lest / so that not"?

This usage generally appears in verbs of fear, caution, or intention, and often introduces a subjunctive verb (فعل مضارع منصوب).

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📌 Example Meanings of "أَنْ":

Arabic	English Meaning
أنْ يَسْقُطَ	lest it falls / so that it does not fall
أنْ يَهْلِكُوا	lest they perish / so that they do not perish
أنْ يَفْهَمَ	so that he may not understand

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✅ Quranic Examples

1. Surah At-Tawbah (9:64)

يَحْذَرُ ٱلْمُنَٰفِقُونَ أَن تُنَزَّلَ عَلَيْهِمْ سُورَةٞ
The hypocrites fear lest a surah be revealed against them...

• "أَن تُنَزَّلَ" = lest it be revealed

• "أن" introduces a negative possibility they fear.

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2. Surah Al-Kahf (18:19)

...وَلَا يُشْعِرَنَّ بِكُمْ أَحَدًا
(Be cautious...) so that no one becomes aware of you.

While "أن" is implied here, the context reflects the "lest" / "so that not" usage.

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✅ Classical Arabic / Pre-Islamic Usage

In pre-Islamic poetry and prose, this meaning is very common:

خَشِيتُ أَنْ تَضِلَّ النَّاقَةُ
I feared lest the she-camel gets lost.

• "أَنْ تَضِلَّ" = lest it strays

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🧠 Grammar Rule

When "أَنْ" expresses negative intention / prevention, the verb after it is:

• Present tense (مضارع)

• In the subjunctive mood (منصوب)

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✅ Conclusion

Yes — you are absolutely right:

"أَنْ" may be used for "so that not" or "lest"
Especially in contexts of fear, intention, or prevention.