Correct use of would

[Romana]

Test No. errors/advan-1 "Give me the bill", question 8

Whenever they would need additional supplies, they used to contact the main warehouse.

(a) would need
(b) supplies
(c) used to
(d) warehouse

Test No. errors/advan-1 "Give me the bill", answer 8

Whenever they needed additional supplies, they used to contact the main warehouse.

Correct entry: needed
The error was: (a) would need

You have not found the error.
Whenever they would need additional supplies, they would contact the main warehouse.
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I have a question concerning the use of "would" for expressing a past event in general. I have heard from several sides that it is grammatically correct to use it. However, my English is very influenced by American slang expressions, so I'm still wondering about the use of "would"... Is it only wrong for this sentence, or wrong in general? Thanks for your help.

[Alan]

In this sentence the if part requires the Past form (so-called imaginary Past) and the second part requires the would form as the sentence is an example of the second (probable) conditional. We also use would in indirect/reported speech when we are converting a future form from direct speech and so He said: I will come changes to He said he would come.
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[Romana]

Yes, I knew these rules. What I meant, however, was if it is grammatically correct to use would as a normal past tense, like in the following sentence:

"When I was a baby my mother would sing a lullaby for me every evening before I went to sleep."

[Alan]

What you have quoted is another use of would, which has a literary flavour rather than a spoken use. It has the meaning of used to but no longer. Very often it is used in the sense of nostalgia. I don't think it's a question of grammatically correct but simply a different use.
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[Romana]

Yes, that makes perfect sense. Thank you very much for your help.

[Adam9092816]

The correct answer is listed as "Whenever they needed additional supplies, they used to contact the main warehouse." I have less of a problem with the "would need" than the "used to". The sentence has established itself in the past tense already and would be better served omitting the "used to". For example: Whenever they needed additional supplies, they contacted the main warehouse. Or even: Whenever they would need additional supplies, they would contact the main warehouse. The "used to" isn't a contrast to another current condition where it would be needed. For example: Though the office now contracts with OfficeMax, previously whenever they needed additional supplies they used to contact the main warehouse.