Something I find often editing my own work, as well as when I critique—when do you refer to an object with "the" or "a/an"?
We use the "definite article" THE to mean a sure, certain, particular thing.
If you just introduced the article in the immediately preceding sentence, THE fits.
If the object is key to the topic, not incidental, use THE.
We use the "indefinite article" A/AN to refer to a general thing.
If you haven't just referred or introduced the object, the indefinite works better.
If the object isn't important: He sat on any-old chair, not the chair—use A/AN.
When in doubt, I suggest:::A/AN
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