the basic rule is that you write i before e. the exception to that rule is that you don't do this after a c (or when the vowels are pronounced like i or a [as in height or weigh]). the exception to this exception is that you still write i before e after a c if they are followed by an n (or if the word is 'weird' or 'foreign' or 'leisure' or 'seize' or maybe 'neither', if you speak an inferior us-american dialect and pronounce it wrongly). the exception to this exception to the aforementioned exception is that certain words with romance or latin origins, like ceinture, enceinte or fluorescein, don't obey the i after c before e before n rule. and, of course, compound words like priceindex or confidenceinterval, but that should be obvious.