I can understand the difference between "dar" and "darse" as "dar" (to give) is a ditransitive verb (a verb which has a direct and indirect object).
Le doy un beso. (I give him a kiss.)
Me doy un beso. (I give myself a kiss.)
But what is the difference if the verb is monotransitive (only has a direct object). For example, what is the difference between "inventar" (to invent) and "inventarse"? Is it:
(Yo) Invento una canción. (I invent (write) a song.)
Me invento una canción. (I myself invent (write) a song???)
It seems to me the only difference is for emphasis? To make things even more complicated, what if the verb is intransitive completely? Take "dormir" (to sleep) as an example. As it is an intransitive verb, shoudn't the fact that the action is reflected upon the person committing it be obvious? For example:
Yo duermo. (I sleep)
Me duermo (???)
Shouldn't the person be the only one committing the act of sleeping on themselves? You can't "sleep" someone else, can you?
To sum up, my questions are about the difference in meaning between verbs and their reflexive form when the verb is ditransitive, monotransitive and intransitive.
Note: I am only a beginner at learning Spanish. If you can, please kindly translate your answer to English! I can hardly read complex Spanish sentences, let alone entire paragraphs! My utmost appreciations to anyone with an answer. Thanks in advance!