Why not “A rat is in my room”, but “The rat is under the bed” is OK.
I think the use of in/definite articles cannot be explained well without explanation of their roles in sentences.
“There’s a rat in my room.” is OK. Or, “I have a rat in my room.” However, in a story or playscript, “A dog walks into a bar.” is OK.
I’m reading a book by Teramura Hideo where he says the first one (“A pen is on the table”) is only OK in play directions.
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