Re: Where using "a" and where "the"?
I can't speak for most native speakers. I never struggle with things like the examples here, and we don't spend much time thinking about them in speech. However, I write for a living and therefore, I spend a lot of time with my written words despite how casual I am in my speech.
Recently I wrote something like this: One of the keys to the success of the project was the extensive communication with the stakeholders.
And it became something like this: One of the keys to the project's success was extensive communication with stakeholders.
Or maybe it didn't -- maybe one of the thes was left in. But the "the" before "communications" referred to the communication specific to this project. And with the modifier, the "the" seemed to work.
Meanwhile, the "the" before "stakeholders" clearly limited it to the stakeholders in this project. I think I left that one out because why bother with the restrictive "the" when clearly the only stakeholders I would write about were the ones involved in the project?
It changed again before it was done. (Writing has many permutations before it's actually finished - and THEN the editor gets it and it changes again!)
It may have ended up as "Extensive stakeholder communication was essential for the success of the project." -- No articles at all.
I don't offer these as particularly great examples of my writing -- they are not -- but to show you that sometimes the article is necessary and sometimes it's not, and sometimes no one else but the writer (and editor) will care, and sometimes it shifts the meaning a tiny bit, and sometimes all it does is add words.
So don't worry if you struggle with these. When you speak, it will come naturally -- or not -- but no one will stop you and say "Wait. Did you mean...?" And when you write, realize you are not alone in wondering which is the best choice.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.