Quote Originally Posted by John_Douglas
you use the ending with "oj" automatically in all words. The only exception is adjectives ending in "sh,shch,zh,ch,ts" - for the words ending in those letters, you use "oj" when the ending is stressed and "ej" otherwise.
Not exactly. "-ой" is used if the sound before ending is hard "-ей" is used if the sound before ending is soft. Exception is adjectives with endings after жчшщц - see Rule 3.

http://www.russianforeveryone.com/Ru...ramUnit7_1.htm

Remember that native speakers are usually not aware of these rules and use them intuitively.