How to use ‘a’ and ‘the’ in sentences

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.