Hi,

During the period from June to December last year prices had fallen. (my tense)

But the book (Focus on IELTS) says, I should write:

During the period from June to December last year prices fell. (book's tense)

I want to ask, is my tense wrong?

what about these two:

By the year 2000, prices will have fallen. (my tense)

By the year 2000, prices had fallen. (book's tense)

For nearly twenty years now prices are falling. (my tense)

For nearly twenty years now prices have fallen. (book's tense)
These are correct:

During the period from June to December last year prices fell. (book's tense) I want to ask, is my tense wrong?-- There is no call for past perfect, which requires the presence of two different past actions.

By the year 2000, prices had fallen. (book's tense)-- 2000 is in the past; it is 11 years ago! Two past actions are mentioned: the fall of prices and the arrival of the year 2000.

For nearly twenty years now prices have fallen. (book's tense)-- 'For...' indicates a duration of time, so use a perfect verb form. 'Prices have been falling' would be even better, probably.
Thanks Mister Micawber

now, I have other questions

can I say : By the year 2020, prices will have fallen. OR By the year 2020, prices will fall. which one?

I thought the third sentence (For twenty years now ...) describes a temporary action so I used present progressive. You mean whenever a duration of time is mentioned in a sentence we have to use a perfect verb?

Your answer was very helpful to me. I really appreciate your help. Emotion: happy
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can I say : By the year 2020, prices will have fallen. OR By the year 2020, prices will fall. which one?-- Either is possible.

I thought the third sentence (For twenty years now ...) describes a temporary action so I used present progressive. -- I don't know what 'temporary' has to do with it, but I would hardly call 20 years temporary in any case.

You mean whenever a duration of time is mentioned in a sentence we have to use a perfect verb?-- No, but for duration from past to present, as here, it is needed. 'Whenever' is not a word we use much in discussing English grammar.