Russian has a few rules and conventions for which vowels can be used with certain consonants.
| ы cannot follow the consonants г к х ж ч ш щ Instead, use и |
я cannot follow the consonants г к х ж ч ш щ ц Instead, use а |
ю cannot follow the consonants г к х ж ч ш щ ц Instead, use у |
unstressed о cannot follow the consonants ж ч ш щ Instead, use е |