grammatical scenarios

I often see myself forgetting how words should be connected together, and I get terrified. I see myself asking whether to follow the term “purpose” with the preposition “of” or “from”; so I compose several illustarctive scenarios and listen to what sounds correct; and whether the terms “promote” and “motivate” are always exchangeable for one another? That is, “we promote a certain topic” by “motivating a person to understand it.” Such questions leave me perplexed when I think about them. I never learned the rules corresponding to all this; I just picked them along the way as I matured, like anyone would do, the way you could naturally tell if a musical note is off. However, when it is time to teach them and consequently you try to focus on them, you start contemplating bizarre options. What happened to those natural automatic choices? In fact we never had to make a choice. If we asked a natural writer that had acquired her writing skills the way a musician plays music without learning the notes, then she would, like me, wonder about those rules, and will try to simply listen to what sounds right. Probably she will compose the rules after that, in retrospect.