7-letter spelling rule:
After г, к, х, and ж, ч, ш, щ —
never write ы, instead use и.
5-letter spelling rule:
After ж, ч, ш, щ, and ц —
never write an unstressed о, instead use е.
8-letter spelling rule:
After ж, ч, ш, щ, ц, and г, к, х, —
never write я or ю, instead use а and у.