Quote Originally Posted by Antonio1986 View Post
Но я заметил, что есть и одна катигория, у которой множественное число формируется с окончанием -и.
Например: плечо - плечи, ухо - уши
That’s ruins of dual — grammatical number for pairs of things that used to exist in Old Rissian. In modern Russian, there’s no dual, but some words keep dual forms in plural.
Shoulders and ears are paired body parts. Another example is око → очи (velar consonants к, г, х became ч, ж, ш respectively before the dual ending -и).
Masculine nouns in dual have the ending -а instead of normal -ы/и: глаз → глаза, рог → рога, бок → бока, берег → берега. In modern Russian, this ending has also spread to some other (non-paired) nouns.