Hard and soft stems

1. These letters follow hard consonants:
   а, э, о, у, ы
2. These letters follow soft consonants:
   я, е, ё, ю, и, ь
3. In Russian grammar, there are many instances when you will need to drop as letter before you add another letter. The general rule is, if you drop a letter from the hard series vowels (а, э, о, у, ы), you add a letter from the hard series vowels, and if you drop a letter from the soft series vowels (я, е, ё, ю, и, ь), you add a letter from the soft series vowels.
4. Let's look at the example for forming plurals of feminine nouns in Russian.
   
   1. Feminine nouns in Russian end in -а (hard series), or -я (soft series).
   2. The plural endings for feminine nouns in Russian are -ы (hard series) or -и (soft series).
   3. So, if the noun ends in -а (hard series), to form the plural we drop that letter and add the letter -ы (hard series): газета - газеты.
   4. If the noun ends in -я (soft series), we drop that letter and add the letter -и (soft series): тётя - тёти.
5. The spelling rules always take precedence over any other rule!
6. Let's look at an example. The word книга ends in a the hard series vowel -а, and you would expect the plural ending to be -ы, since it is a hard series vowel as well. However, Spelling Rule 1 takes precedence, and therefore the noun will take the plural ending -и.