The Russian Spelling Rules

1 - the 7 consonant rule

ы cannot occur after the sibilants (ж, ч, ш, щ) or velars ( г, к, х), and must be replaced by и

e.g. студенты - students, but студентки - female students

2 - the 8 consonant rule

я and ю cannot occur after the sibilants (ж, ч, ш, щ), velars (г, к, or х) or ц, and must be replaced by а and у

e.g. говорю, говорят, but лежу, лежат

3 - the 5 consonant rule

after the sibilants (ж, ч, ш, щ) or ц, words are spelled with o if that syllable is accented and e if it is not

e.g. в большoм хорошем доме - in a big nice house


More examples at: Using Russian 7.2.4; Dr Beard's Online Reference Grammar