That's just the way it is

I love English. It's a strange, enormously flexible, beautiful and accomodating language. But explaining it is difficult. I hate it when it happens, I try to avoid it whenever possible, but all too often all I can tell my students is "That's the way it is". The more advanced students are used to this, but the confusion on their faces at the lower levels can be heartwrenching. Why do we say "under the law"? How come "can" doesn't have an infinitive, perfect or future form, but we can use it to talk about the future anyway? Why can't you say "very furious" only "really/absolutely furious"? Why do we use the past to talk about hypothetical situations in second conditional sentences (If I won the lottery, I'd buy a big house)? Why aren't there any consistent rules for negative prefixes? Irrelevant, but unreliable or disrespectful. Immature and immoral, but unmanageable. Why do we use the present simple for routine actions (I go to work every day) and not the continuous (I am going to work every day)? I have no answer. That's just the way it is. There's a metaphor here somewhere, but I can't quite reach it.