Fill vowels

1. The fill vowels -e- and -o- are often added to separate the last two consonants of feminine and neuter nouns, when the last vowel is dropped to form the genitive plural:

   nom. sg.	genitive pl.
   окно	окон
   кухня	кухонь
   игрушка	игрушек
   девочка	девочек

2. The rules concerning which of the two vowels to use are complex. Below are few simple explanations.
3. If the first of the two consonats is к, г, х, insert the vowel -o-:
   oкно - окон
   кухня - кухонь
4. If the second consonant is к, г, х, insert the fill vowel -o-:
   остановка - остановок
   байдарка - байдарок
5. Obviously, you would need to make sure that the first vowel does not violate a spelling rule, in which case you put the fill vowel -e-:
   игрушка - игрушек
   девочка - девочек
6. In all other instances you insert the fill vowel -e-.