usage of THAN, confusion, please help, thanks!

[tolotolot247]

Hi, instructors, I am always confused about the usage of than. Could you please help me?
Are the following sentences right or wrong, and why?

1. Gas prices in 2012 are higher than in 1997.

2. Gas prices are higher in 2012 than in 1997.

3. There are more books in box A than in box B.

4. There are more books in box A than books in box B.

5. There are more books in box A than those in box B.

6. There are more books in box A than them in box B.

I also have some other questions.

3. There are more books in box A than in box B.

What does the prepostional phrase" in box A" modify?

5. There are more books in box A than those in box B.

What does the pronoun"those" refer to?

6. There are more books in box A than them in box B.

What does the pronoun"them" refer to?

Thanks in advance!

[Sage Pearce-Higgins]

I'd encourage you to organize your study around real GMAT problems (or questions from our strategy guides). If you try to master all the rules of English your studies will take a long time. Let me try to help you:

3. There are more books in box A than in box B.
This is a totally fine comparison: it's clear what's being compared, and the fact it leaves out a few words is typical efficient English. We could say 'There are more books in box A than there are books in box B.' but we don't need to. The phrase 'in box A' modifies (i.e. gives extra information about) 'books'.

4. There are more books in box A than books in box B.
This is a bit weird, most likely incorrect. We'd be more likely to repeat the noun if the nouns were different. And we're not actually comparing the books, we're comparing how many books there are, so we'd need 'there are'. For example: There are more books in box A than there are toys in box B.

1. Gas prices in 2012 are higher than in 1997.
This is probably incorrect. Although I think the meaning here is clear (we're comparing gas prices), GMAT tends to avoid similar constructions because of the risk of ambiguity. There's also the a tense issue, since 1997 is in the past. Check out SC 758 (OG 2018) but notice that there are other reasons to eliminate answers A and C.

2. Gas prices are higher in 2012 than in 1997.
Same as above.

5. There are more books in box A than those in box B.
Incorrect. Since we're comparing the number of books that there are, 'those' makes an unfair comparison. A correct example would be: The books in box A are bigger than those in box B.

6. There are more books in box A than them in box B.
Totally incorrect, both for the reason given for (5), and for the fact that them refers to the books in box A, making a paradox.