Here are two example sentences from the start of a fairy tale. Note how the hunter is first introduced with ga (が), and in the second sentence ga (が) is replaced with wa (は). 

昔々、狩人が住んでいました。(Once a upon a time, there lived a hunter.)
狩人は、毎朝山を登りました。(Every morning the hunter climbed up the mountain.)

N.b. unlike in these two sentences the difference is not always akin to the English distinction between "a hunter" and "the hunter".
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