But for, were it not for and lest

But for your help, were it not for the rain, lest we forget: formal ways to say "without" and "in case".

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Formal English has short ways to say "if something did not exist or had not happened": but for + noun, and the inverted forms were it not for and had it not been for. Lest means "so that ... not" or "in case". All of them belong to formal writing, speeches and literature; in everyday English, if it wasn't for, without and in case are more usual.

Formal	Everyday
But for your help, we'd have failed.	Without your help, we'd have failed.
Were it not for the rain, ...	If it weren't for the rain, ...
Had it not been for the delay, ...	If it hadn't been for the delay, ...
Speak quietly lest you wake her.	Speak quietly so that you don't wake her.

But for + noun

But for is followed by a noun phrase or a pronoun, never by a clause; to add a clause, use but for the fact that. The main clause has would, could or might, and its tense shows whether you mean now or the past.

But for the bad weather, the event would have been a great success.
If it hadn't been for the bad weather, ...
But for his pension, he would have nothing to live on.
If it weren't for his pension, ...: a present meaning.
But for you, I might never have applied for the job.
Incorrect: But for you helped me, I would have failed.
Correct: But for your help, I would have failed. / If you hadn't helped me, I would have failed.
But for takes a noun, not a subject and a verb.

But for has a second, non-conditional meaning: "except for". Then the main clause has no would.

The street was empty but for a few parked cars.
Except for a few parked cars.

Were it not for and had it not been for

These are inverted conditionals with it as a fixed subject. Were it not for is about now or always; had it not been for is about the past. The two halves can refer to different times, as in a mixed conditional.

Were it not for the volunteers, the library would have to close.
Had it not been for the loan, the business would have closed last year.
Had it not been for that teacher, I wouldn't be a writer today.
Past condition, present result.
Incorrect: Were not it for the volunteers, the library would close.
Correct: Were it not for the volunteers, the library would close.
In the inverted form, not comes after it.
Incorrect: Were they not for the volunteers, the library would close.
Correct: Were it not for the volunteers, the library would close.
The subject is always it, also before a plural noun.

Lest + base form or should

Lest introduces something you want to avoid. It is followed by the base form of the verb (a subjunctive) or, especially in British English, by should + base form. It is already negative, so do not add not.

He kept his voice down lest anyone hear him.
She checked the door twice lest someone should break in.
Lest we forget.
The words on many war memorials: so that we do not forget.
Incorrect: Write it down lest you don't forget it.
Correct: Write it down lest you forget it. / Write it down so that you don't forget it.
Lest already means "so that ... not".

After words of fear, lest means "that": There was a fear lest the bridge collapse. This sounds old-fashioned; today people say There was a fear that the bridge might collapse.

Everyday equivalents

Outside formal writing, choose the plainer forms. They are always correct.

If it wasn't for the rain, we'd go out.
Informal; if it weren't for is preferred in writing.
If it hadn't been for the delay, we'd have caught the train.
Without her advice, I'd have made a terrible mistake.
Take a map in case you get lost.
I wrote it down so that I wouldn't forget it.
Leave now, otherwise you'll miss the bus.