You are actually discussing two different topics.
One topic is learning a language, where a person learns real words like τοῦ γράφοντος, τὸ λαλοῦν, τὸν αγαπῶντα
All ancient Greeks learned the language from real words. Presumably, the goal of language learning is to develop the highest fluency in the shortest time. To do so requires real language use with real words and without outside processes.
the other topic is the history of how words were developed. The above words came via
τοῦ γράφοντος, τὸ *λαλέον, τὸν *αγαπάοντα
You can see the -ντ- history in some of the remaining forms, and you can also see the interactions of various vowel endings -ε- and -α-.
However, it is very important to learn real words in context. If a person learns "rules" they will teach their brain to leave real language mode and enter an outside-language analytical mode when reading Greek. Eventually, they will learn to do this very fast, but it will still have the effect of wiring the language into a non-language framework in the brain. This is not the path to fluency in a language. It is a proverbial "short road that ends up the longer road"
Instead, a person must use real words in real contexts in order to build fluency. Then, with the language infrastructure in place as a functioning language, a person can discuss the history to one's content and at whichever level one desires. This is the "long road that ends up the short road."