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"Does the word before in the following sentences have the same meaning as the word lest?"

			You have to leave now before you miss the last train.
			You have to hurry up before you miss the train.
			I have to give him $10,000 before he tells the police that I was there.
			We have to bribe her before she talks.
			You have to do all you can before she leaves you.
			We have to give them the money before they continue blackmailing us.
			We have to introduce this computer before we waste a lot of time and energy.

The Cambridge Online Dictionary gives the following definition for lest:

lest
conjunction
LITERARY
in order to prevent any possibility that (something will happen)

			They were afraid to complain about the noise lest they annoyed the people next door.
			Lest you think the film is too violent, I must assure you that it is not.

In Practical English Usage - New Edition (Oxford University Press, 1997), Michael Swan explains:

"Lest has a similar meaning to in case or so that … not. It is very rare in modern British English, and is found mostly in older literature and in ceremonial language. It is a little more common in formal American English.

			They kept watch all night lest robbers should come.
			We must take care lest evil thoughts enter our hearts.

Lest can be followed by a subjunctive verb.

The government must take immediate action, lest the problem of child poverty grow worse.

For fear that is used in a similar way, and is also unusual in modern English.

He hid in the woods for fear that the soldiers would find him."

p. 305

So, as far as the examples provided by our reader are concerned, whether or not before can be substituted by lest depends on how the speaker sees the action or event that s/he is trying to avoid. If s/he is sure that the action or event is going to take place, then before is better. If s/he is not so sure, then lest is better. Thus:

			You have to do all you can before she leaves you.
			You have to do all you can lest she leaves you.

In the first sentence, the speaker is sure that she is going to leave, whether or not the other person does all he can, and therefore before is better. In the second sentence, the speaker believes that doing something can prevent her leaving, and so lest is better.

The same criteria (how certain the speaker feels) can be applied to the other examples.

To see many examples of the use of lest, see the web concordancer. Type lest into the 'search string' field, select any corpus in the 'select corpus' field, and then click on the 'search for concordances' button.

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