Using "Before" with gerunds

Question

Consider this sentence:

M couples leave a party... What is the number of men before couples leave

What is the proper word for the bold part:

• Before couples leave
• Before leaving of couples
• Before that couples leave
• Before leaving by couples

I know after "Before" and "After" usually a gerund comes, but I can't guess how can I write the sentence to include a gerund. Anyways, which case is correct or incorrect and why. Should I always repeat the previous sentence "They leave"? What if it was a long sentence?

Answer 1

While you can follow before or after with a gerund, it's not a rule by any means. In this specific case, there is no need whatsoever to use gerunds.

I would simply say:

M couples leave a party... What is the number of men before the couples leave?

You could also say:

M couples leave a party... What is the number of men before the leaving of the couples?

However this is a little more verbose and awkward than the first, but it does use a gerund.

Answer 2

Since you ask about the length of the sentence, it has no bearing on the question posed afterwards, except insofar as the questioner would wish to repeat something stated about the subject.

Five blindfolded couples arranged in a quincunx on a 50' x 50' dance floor are told to foxtrot until they collide with another couple. Assuming normal foxtrot tempo, how much time elapses ___________________.

A. before a colliding of two (blindfolded foxtrotting) couples
B. before a collision of two (blindfolded foxtrotting) couples
C. before two (blindfolded foxtrotting) couples collide
D. before that two (blindfolded foxtrotting) couples collide

D. is ungrammatical.
A. is awkward and unnatural but grammatical
B. is needlessly abstract, but grammatical
C. is idiomatic/natural