I want to ask some questions about this sentence.

[CHUNX638]

Despite a growing population, in 1998 the United States used 38 billion fewer gallons of water a day than it did during the period of all-time highest consumption almost 20 years earlier.
This sentence is from Gmatclub.

I want to ask some questions:
1. I notice its a comparison. The comparison marker in this sentence is X fewer than Y. I want to know what the two elements.
2. Can you help me to add the omitted words in the Y element?
3. "in 1998" modifies what?
4. "during..." modifies what?

Thank you very much!

[Sage Pearce-Higgins]

Please read the forum guidelines before posting: it would be great to have the full problem, and the source (although it was posted on GMAT Club, it would be good to know which book it comes from; if you don't know which book it comes from then I strongly suggest that you ignore this problem, as there are many poor-quality GMAT resources available).

In answer to your questions:
1. I notice its a comparison. The comparison marker in this sentence is X fewer than Y. I want to know what the two elements.
Perhaps if I rewrite the sentence in this way, the comparison is clearer:
in 1998 the United States used 38 billion gallons of water a day fewer than it did (during the period of all-time highest consumption) almost 20 years earlier.
2. Can you help me to add the omitted words in the Y element?
It looks like 'it did' means 'the United States used'
3. "in 1998" modifies what?
The X element.
4. "during..." modifies what?
The Y element.

[CHUNX638]

Hi, Sage!

in 1998 the United States used 38 billion gallons of water a day fewer than it did (during the period of all-time highest consumption) almost 20 years earlier.

So it compare a time point(1998) with a period(during the period of all-time highest consumption), this comparison is right?

And I want to ask:
what "almost 20 years earlier" modify? "it did" or "during the period of all-time highest consumption"?
Thank you!

I'm sorry I don't know which book this sentence comes from, and next time I'll do some questions which come from a book.

[Sage Pearce-Higgins]

The comparison phrase here is "38 billion fewer gallons of water a day than", so the sentence is comparing the amount of water used in the different time periods. It's totally fine to compare 'in 1998' with '20 years earlier'.

Examine these (correct) examples:
John ate more donuts than Sajid ate.
John ate more donuts than Sajid did.
John ate 5 more donuts than Sajid did.
Yesterday, John ate 5 more donuts than Sajid did the day before.

As for the modifier issue, I'd say that 'almost 20 years earlier' modifies 'during the period of all-time highest consumption'. However, this is a moot point: you don't need to undertake exhaustive grammatical analysis of a sentence.