"a(n)" is an indefinite article, so it is used with determined things. and "the" is a definite article, so it is used with indetermined things. I'll give you some examples. 1. There are two apples. One is green, the other is red. --since you've already mentioned one apple(the green one), there's only one left(the red one). so, because it has already been determined, you use "the other". 2. There are three apples. One is green, another is red and the other is yellow. --when you've already mentioned one apple, the left two apples are an undetermined "another" and the determined "the other". 3. There are three apples. One is green, the others are red. --these sentences mean "one apple is green and the others are all red". since the red ones are determined and plural, you use "the others". 4. There are lots of apples in my basket. One is green, others are red and the others are yellow. --when you don't know how many apples there are and you've already mentioned one(the green one), some others(donno how many) could be "others" and the rest(donno how many) are "the others". and when green apples are also plural(neither know how many), you can say "Some are green, others are red and the others are yellow." hope it helps!! :]