I know there are quite a few threads about in case vs. lest, but I think none has seriously questioned the presumed interchangeability of the following:

  1. Take an umbrella with you in case it rains.
  2. Take an umbrella with you lest it (should) rain.

1 is definitely idiomatic, but 2, according to the dictionary definition of lest, seems problematic, even though some native speakers regard it correct, however uncommon lest is.

lest means "so as to prevent something from happening." (The other use basically means "that" and is used following adjectives denoting fear, worry, etc., and is outside the scope of the present discussion.)

But having an umbrella with you will not prevent the rain from happening. If so, would you still consider the use of lest in 2 correct?

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    The word lest is becoming increasingly uncommon (particularly in spoken contexts), so I wouldn't spend too much time worrying about how to use it. Partly because it's a dated usage, you may see it with the equally old-fashioned subjunctive verb form (lest it rain). But native speakers don't like the subjunctive much, so often we'd either "disguise" it with should, or substitute an inflected verb (rains, in your case). – FumbleFingers Reinstate Monica Sep 9 '17 at 14:36
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    What you are 'preventing' when you take your umbrella is not the rain but implicitly your getting wet as a consequence of the rain. – StoneyB on hiatus Sep 9 '17 at 14:39
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    Note that your definition lest = so as to prevent something from happening is just one way of summing up what the word means, and how it can be used. The full OED has in its first definition introducing a clause expressive of something to be prevented or guarded against. – FumbleFingers Reinstate Monica Sep 9 '17 at 14:42
  • What @Fumble said about disguising it with should (or normal present or past tenses) I believe is restricted to BrE. In AmE, the subjunctive is much more frequently used: it remains in fixed phrases as neutral, and in other constructions as neutral/formal. – user3395 Sep 9 '17 at 14:47
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    @Andrew I don't think should sounds that pompous in BrE as it does in AmE, in fact, it's a standard substitution for the subjunctive (as far as I know and from what I've heard/seen). – user3395 Sep 9 '17 at 15:04

You cannot pick out one definition from one dictionary and call it "the" dictionary definition. Here we should insist on the indefinite article there.

For some 800 years now, and more, the word lest has been used to mean "for fear that" something will happen or "to guard against" something happening.

You'd better tie a double knot in that lest it come undone.

You'd better set your alarm, lest you oversleep.

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  • Both of your examples are irrelevant, as they are instances of the (unquestionable) sense of "so as to prevent something." – Apollyon Sep 10 '17 at 11:55
  • @Apollyon: Why are you insisting that we take its meaning as "prevent" in your rain example? It's not my answer that is irrelevant but your question. "But having an umbrella with you will not prevent the rain from happening. If so, would you still consider the use of lest in 2 correct?" – Tᴚoɯɐuo Sep 10 '17 at 11:57
  • The examples are irrelevant, as they concern a sense of lest that is not under discussion. – Apollyon Sep 10 '17 at 11:58
  • The relevant sense of lest is, as FumbleFingers rightly pointed out, "so as to guard against something," But your examples belong to the sense "so as to prevent something." – Apollyon Sep 10 '17 at 12:01
  • @Apollyon: Your question asks whether lest is incorrectly used with rain because we cannot prevent the rain. But why does your mind even go there when one of the valid meanings is "for fear that it rain"? Why focus on the meaning that does not apply to the situation when another meaning does apply? – Tᴚoɯɐuo Sep 10 '17 at 12:05

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