The following sentences are taken from MGMAT/SC/P101
We have the comparative structure in the following sentences:
1. The tycoon contributed more to the candidate's campaign than did anyone else in the industry.
2. Three times more students attended the prom this year than did last year
Then we have a different comparative structure in the following sentences:
2. The blue dress looks more flattering on you than the red one does.
3. Sam was away on vocation longer than his friends were."
Why can't we use in the second and the third sentences the same the structure as in the first sentence, changing them as follows?
1. The blue dress looks more flattering on you than does the red one"
2. Sam was away on vocation longer than were his friends."
What is the rule here? Thanks a lot for your help.