I'm tutoring a Russian student in English, and she has a grammar question that I can't answer. (Not the first!)
When you write comparisons, when do you add the verb for the second subject, as shown below.
1. The number of students is greater this year than last year.
2. The number of students is greater this year than it was last year.
This is not the best example, but it's the first one I could come up with. Is there a rule for this, or is it writer's choice?