Lest means so that… not, or for fear that. She wrote down the address lest she forget it: she didn't want to forget. The 'not' is already built into lest, so no second not is needed. After lest comes a base verb or should: lest she forget, lest she should forget. It's bookish and solemn: Lest we forget.
How would you use lest?
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action + lest + subject + base verb; lest + subject + should + base verb
- She wrote down the address lest she forget it.
- We spoke quietly lest we should wake the baby.
- He checked the label twice lest he send the parcel to the wrong office.
What changed
First versionShe wrote down the address lest she should not forget it.
RevisionShe wrote down the address lest she should forget it.
The idea is 'so that she would not forget', so not seems necessary. But lest already means 'so that… not', and an extra not reverses the meaning: lest she should not forget means she's afraid of not forgetting. Remove not, and it makes sense again.
So that… not says the same in everyday English: She wrote it down so that she wouldn't forget. In case means 'because it might happen': Write it down in case you forget. Lest sounds bookish and is rare in conversation.
Your turn
She wanted the baby to stay asleep, so she whispered lest she should ___ him.
Show the answer
wake — The unwanted event is waking the baby, so after lest she should you need wake. Not wake would mean she's afraid of not waking him.
Retell, in the style of an old fairy tale, a precaution the hero took. Use lest.
He hid the letter lest anyone should read it before morning.
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