'A' or 'The': 'A long time ago two friends were passing through a forest.'

[Tom]

Hi

A teacher narrates this story to his students. The story with with the following line. Could you please tell me if it should be "a forest" or "the forest"?

[Yankee]

Hi Tom

You should use 'the' only if which specific forest has already been mentioned or if the listener/reader can be expected to know somehow which forest you're referring to.
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