HI. I came across this sentence in a grammar book and it seem that the past perfect is used to express an action later than the main verb of the sentence rather than before it.

"The plane took off before they'd completed the security checks."

I think this sentence means, "When the plane took off, they hadn't completed the security checks." but why is the past perfect used in the latter clause? Why isn't it " The plane took off before they completed the security checks*," , or even "The plane *had taken off before they completed the security checks," which seems to use the past perfect correctly (at least in terms of the order of events...)

Thanks everyone for your help
Your sentence is easily understood. However, I contend that before and after imply that the action was completed. I would change this sentence to say: The plane took off without the completion of the securty checks.
hi philip, thanks for your reply, however i don't think you've answered my question. My question is-- why is the past perfect used in the original sentence, when the action described by the past perfect actually occurred after the main verb rather than before it.

The plane took off before they had completed the security checks.
(earlier event) (later event)

As is my understanding of the past perfect, it should be used to describe the action that comes earlier -

The plane had taken off before they completed the security checks.
(earlier event) (later event)

Swan has an example in 97:3: with "he went out before i had completed the sentence"
(earlier event) (later event)

I'm simply curious to know what the theory is behind the seeming inversion of a tense that is supposed to designate an earlier action. Is it perhaps something to so with the fact that the later action should have been (or was expected to be) completed before the earlier one, but in fact was not completed at all?
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Anonymoushi philip, thanks for your reply, however i don't think you've answered my question. My question is-- why is the past perfect used in the original sentence, when the action described by the past perfect actually occurred after the main verb rather than before it.
Sorry my thoughts weren't more clear. I don't think the past perfect should have been used at all (and that isn't your fault).
The past perfect is correct. The security checks started before the plane took off.