1. In my salad days, I experienced the problem of frequency of payment firsthand.
2. If the general frequency of payment changes in a society, this will shift the
demand for money and raise or lower the price level.
3. On the other hand, if frequency of payment of salaries should shift generally from once a week to twice a month, the reverse will happen.
4. Frequency of payment is not going to go up or down every year.
1) 'of' is used because it is the problem (the problem 'of' frequency of payment)
2)' The' simply states it is specific, not 'a' frequency of payment, ie not 'a cat', but 'the cat'
3) 'if' is used because we are talking about possibility (if.....should shift, not did...shift...or will ...shift, (so uncertainty)
4) Is simply a statement or opinion