What's the difference between a verb and their reflexive form if the verb is mono/intransitive?

Question

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).

For example:

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!

Answer 1

Some of these are aspectual datives (i.e., not reflexives), which are indeed used for emphasis. They are explained in this canonical answer. For verbs of "consumption" (comer, beber...), the dative pronouns (me, te, se) emphasize that an item was consumed completely:

• Se ha bebido una botella de cerveza ("He has drunk an entire bottle of beer")

For inventar, using the verb without a dative pronoun has a more positive meaning ("invent"), while the dative form is used more in the sense of "making something up":

• Alexander Graham Bell inventó el teléfono ("Alexander Graham Bell invented the telephone")

• Se ha inventado una historia para justificar su ausencia ("He has made up a story to justify his absence").

For dormir, there is actually a reflexive (not dative) use, as explained in the DLE. Dormir is "to sleep":

1. intr. Hallarse en el estado de reposo que consiste en la inacción o suspensión de los sentidos y de todo movimiento voluntario.

and dormirse is "to fall asleep":

13. prnl. Quedarse una persona o un animal dormidos.

The abbreviation "prnl." in the DLE indicates a reflexive (pronominal) use of the verb.