I'm a beginner.i was trying to read a jlpt n3 reading file.i understand almost all except for this phrase i cannot figure out its meaning: その ホテル では,客 が チェックアウト した あと も 部屋 の ゴミ は もう 一泊 させる の だ そう だ.
The core of the sentence is そのホテルでは、部屋のゴミはもう一泊させるのだそうだ。.
1) Have you already learned the function of ～そうだ at the end of the sentence?
2) Have you already learned the causative sentence? What is the causee in 部屋のゴミをもう一泊させる?
Yes, that's right. Both are correct. The sentence uses the personification. The causee is the trash, and 部屋のゴミをもう一泊させる "they(= the hotel) let the trash of the room stay one more night" actually means "they keep the trash(= they don't throw the trash away) for one more night".
Because you wrote "All the sentence that I wrote above is what I don't understand." You actually already got the meaning of the clause? If so, you would be able to understand the whole sentence, no?
Can you give us your translation of the whole sentence? This is a general idea and not just to you, but we often see the cases that "learners think they understand the meaning of a sentence" is not the same as "their understanding is correct". I believe it's useful for you to confirm whether you really understand it correctly or not.