You don’t need a reason why, you just need to know that’s that’s how people — natives — say things. That’s more than enough for you do to deal with right now. Why create work for yourself?
Just copy. Crib. Don’t “show your work”. Cheat. Go straight for the right answer, no ifs ands or buts.
About the only time I have found knowing reasons why to be remotely useful on a consistent basis is with punctuation. Punctuation seems to need a reason and seems to be relatively consistent about it. A full stop marks the end of a sentence.
Grammar and usage, however, simply works by fiat. It may have a logic, but not one that’s worth you trying to plot explicitly, like the the trajectory of a ball on a graph. Just catch the ball. Grammar and usage is because it is; they are because they are. Anything we invent afterwards is just a comforting myth, a just-so story, maybe even entertaining historical fiction, but that’s all it is.
Too many exceptions means don’t bother looking for a pattern; you’ll just end up “stereotyping” and being wrong; if words were human beings, it’d be prejudice 😛 . Too much argument means the reason why doesn’t matter — because no one really knows. And if you read any academic papers on Japanese grammar, you will find plenty of argument — on the most basic of issues, on issues you’da thought would be settled. In fact it sometimes seems that the simpler and more common the word, the more heated the arguments. Copula this, である that, is です really the same word…
Leave the reasons why to the philologists; you just need to talk proper.
And how you do you do that? Just copy. Crib. Don’t “show your work”. Cheat. Say what natives say. Why is it right? Because they say it that way, that’s why. That’s all the right and why you need, that’s all you handle right now. People are always complaining about how “hard” it is to “learn” a language — well why not stop doing stuff you don’t have to do and that won’t help you? It’s a start, right? Stop hauling water up the steps and start using taps.
Stuff is right because native speakers say it (is); it’s a matter of general consensus, not raw, naked reason. An invisible democracy of sorts. Even “wrong” usage eventually becomes right if natives begin to decide it is. And vice versa. Standardized spelling in English is only a couple hundred years old.
But what if you like knowing why? Good. Great. Go for it. But don’t kid yourself that it’s going to help you speak better, because it won’t. It’ll just help you know why. Maybe you can go on Jeopardy with that trivia. It can make you a better pedant and give you something to look down on people for and make them feel inferior about at parties; I know it doesn’t look like it, but I’ve been a dues-paying member of the Grammar Nazi Party, too, you know.
You have so much on your plate already. And it isn’t helping you. Lighten the load. Give it to Samwise. Drop the whys and the grammar tables and have a lighter, funner trip.
Or maybe you like pain…