When, how & why to feed are subjects of great debate & discussion.
A simple addition to sugar & water is Vitamin C (ascorbic acid). ~1500mg/gallon will reduce the pH to near that of honey. Why? Because the "near neutral" pH of sugar syrup promotes a number of hive problems - American Foul Brood, European Foulcbrood, Chalkbrood & Nosema, to name a few.
If you have a pH tester, aim for somewhere in the normal range of honey, 3.2-4.5 . Some also use vinegar but I haven't seen a reliable recipe.
When I've found need to feed, I've used 1500mg of Vit C/gallon.
for more info - see "Ecology of the Hive", http://bushfarms.com/beesfoursimplesteps.htm