Nouns in plural
Nouns are made plural by adding or changing their endings. Masculine and feminine nouns are made plural with the endings -ы / -и.
Neuter nouns are made plural by replacing the endings -о /-е with -а /-я.
The letter -ы never follows к, г, ш, ж, щ, ч, х. After those letters there should be the letter -и (банк – банки, собака – собаки).

There is a list of exceptions where the plural is formed differently. Please memorize them.
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человек – люди
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ребёнок – дети
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друг – друзья
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брат – братья
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сестра – сёстры
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яблоко – яблоки
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дом – дома
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паспорт – паспорта
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город – города
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лес – леса
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цветок – цветы
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цвет – цвета