(b) is more universal, can be said in any situation. default word ordering.
We can emphasize any word during speech without reordering of words, just by tone of voice.
But sometimes semi-rule can be used: place important (emphasized) words in first positions.
So, situations exist where (b)-form is good, but I doubt it can be explained easily. Just read/listen natives and get used to common patterns.
Я начал изучать русский в 2012 году и ещё я не понимаю простые вещи!