Russian Spelling Rules
Here are the so called Russian spelling rules. They are just used to make pronounciation of a word clean when soft consonants might make this difficult. Generally, you will notice that the consonants these rules apply to are the consonants that don't exist in English.
Spelling Rule 1: Ы - И
Never write the letter "Ы" after the letters 'Г, К, Х, Ж, Ч, Ш, Щ' instead use "И"
Spelling Rule 2: unstressed O - E
Never write an unstressed "O" after the letters 'Ж, Ч, Ш, Щ, Ц' instead use "E"
Spelling Rule 3: Я - А
Never write the letter "Я" after the letters 'Г, К, Х, Ж, Ч, Ш, Щ, Ц' instead use "А"
Spelling Rule 4: Ю - У
Never write the letter "Ю" after the letters 'Г, К, Х, Ж, Ч, Ш, Щ, Ц' instead use "У"
There are times when the last consonants of words are changed when the word's ending is changed. Again this is to make them easier to pronounce. When consonant mutation occurs here are the consonant that change.
п - пл
б - бл
ф - фл
в - вл
м - мл
т - ч
к - ч
д - ж
з - ж
г - ж
с - ш
х - ш
ст - щ
ск - щ
Remember that the spelling rules may apply to the vowels that follow.
You will often see consonant mutation take place when conjugating verbs. In particular the 1st person singular, but there are also other times it's used. Here are a couple of examples.Видеть - Вижу (see)
Любить - Люблю (love)
Сказать - Скажу (say)
There is a detailed discussion of this concept at this external link.