Every evening, we play the かたづけ怪獣 song, where they sing:
おかたづけができないと あらあら、大変 (x3) おかたづけ怪獣が出て来るぞ！
Now I've been grappling with this language long enough that ‥ができないと sounds natural in this context, while ‥はできないと sounds off, but I couldn't put my finger on why. My wife, a native speaker, pondered this for a moment and then theorized that は would be a simple statement of fact:
If you can't clean up, the cleanup monster will come!
While が implies that, while there may be lots of things you can't do, surely you can at least clean up:
If you can't even clean up, the cleanup monster will come!
I buy the first half of that explanation, but the second seems fuzzy. Is she right, and is there a more general rule here?