How to make a noun plural
Russian nominative plurals end in -ы, -и, or -а. The ending depends on the last letter and on a famous spelling rule that runs through Russian.
The default endings
Masculine ending in a hard consonant → -ы (стол → столы, кот → коты). Feminine ending in -а → swap for -ы (мама → мамы, книга — but wait, see the spelling rule below). Neuter ending in -о → swap for -а (окно → окна, слово → слова).
The famous spelling rule (7-letter rule)
After the letters Г К Х Ж Ш Щ Ч, you cannot write the letter ы. You write и instead. So: книга → книги (not 'книгы'), друг → друзья is irregular, but кот → коты is fine because т isn't in the list. This rule will follow you through your whole Russian career — memorize the seven letters.
Soft endings
Nouns ending in -я or -е/-ь take -и in the plural. Песня → песни. Море → моря (irregular stress shift, but still neuter pattern with -я).
Irregular plurals to memorize
Some common nouns have irregular plurals: дом → дома (with -а, even though it's masculine), друг → друзья, брат → братья, мать → матери, дочь → дочери, сын → сыновья, ребёнок → дети, человек → люди. Don't try to derive them — just memorize.
