Here are some answers to your questions: 1. What is the difference between 'in the' and 'at'? Are the prepositions of my sentences grammatically correct? 'In the morning' is correct; 'at morning' is wrong. It is normal to say 'in the morning', 'in the afternoon' or 'in the evening'. The only time period which can go with 'at' (and no article) is 'night'. We say 'at night', but we cannot say 'at' with other periods of the day. You can use 'during the' with any period. 2. The third sentence means this: 'He thinks while listening to music in the evening.' But I wanted to express "listening to music" poetically (personified). Can 'by some music' or 'beside some music' be used? No. Sorry. I think you must be trying to express an idea from Japanese which cannot be translated literally. Your third sentence doesn't really mean anything clear. Neither of those preposition work, I'm afraid. 3. I wrote 'something', 'anything' and 'some' to refer to an unspecified object. Is it better to write without? You could say 'He reads something in the morning' and 'He writes something during the daytime', but it isn't necessary. If you want to emphasise the activity, or the fact that he writes/is a writer in the daytime, it's better just to say 'He writes during the daytime'. 'Anything' does not work in this context. I hope that helps.

Also "A thinker about music in the evening" is better but still a bit awkward.