Nouns in Plural / Существительные во множественном числе

For putting nouns into the plural we have to add the letter -ы. The very important rule you need to know is that we never have the letter —ы after these letters:   к, г,  ш, щ, ж, ч, х  (about how to remember them easily read here)

In case if there is one of these letters in the end of a word, the ending is —и (not ы):

• банк – банки (a bank – banks)
• парк – парки  (a park – parks)
• гараж – гаражи (a garage – garages)

The same thing happens if there is the soft mark (ь) or й in the end. But the difference is that we are replacing them (not adding after) with the -и:

• ночь – ночи (a night — nights)
• отель – отели (hotel — hotels)
• музей – музеи (museum — museums)

Regarding the feminine nouns (More about genders here), we are replacing the last vowel with the -ы or -и. 

We replace -а with the -ы, and we replace -я with the -и.

• машина – машины (машинаы)   (a car — cars)
• идея – идеи an idea — ideas

And don’t forget that it is impossible to have the letter -ы after к, г, ш, щ, ж, ч, х. So if the last letter -a follows one of those letters, you should replace it with the -и (not -ы)

• девушка – девушки (девушкы)  (a girl – girls)
• собака – собаки (a dog – dogs)

So about masculine and feminine nouns it is clear. We use endings -ы or -и. But neuter gender nouns make plural in a little different way. For neuter nouns we have to replace the last letter -о with the -а, and if a noun ends with the —е, we replace it  with the —я.

• окно  – окна (a window – windows)
• море  – моря (a sea – seas)