Student or Learner
My student asked me which is better:
the stock of Citibank
I told him that 'Citibank's stock' is better, but I wasn't able to give him a good explanation.
The Department of Defense / The Defense Department
Are there rules for when to use the apostrophe 's' and when to use 'of'?
Are there times when we have to use 'of' and times when we have to use 's'?
Any clarification on this subject would be greatly appreciated.
As a general guide, 's is acceptable in most cases. It can be very cumbersome to use "of the", as shown in your Citibank example.
My own feeling is that you must look at the context and structure of the text and use what is comfortable within that.
With reference to "the Department of Defence" and "Defence Department", one is the formal title of the department, the other is the colloquial alternative.