усн

усн

The word "усн" does not exist in Russian. Perhaps you meant something else?

In this sentence, 'уснул' is the past tense form of the verb 'уснуть,' which means 'to fall asleep' or 'to go to sleep.' It is used to describe the action of someone beginning to sleep.

Младенец наконец уснул.

The baby finally fell asleep.

Here, 'сон' means 'sleep,' referring to the state or activity of sleeping, and is a noun derived from the same root 'усн' related to sleeping.

Её сон был крепким и безмятежным.

Her sleep was deep and serene.

This sentence uses the infinitive form 'уснуть,' showing the potential or possibility of the action of falling asleep.

Перед путешествием ему трудно было уснуть из-за волнения.

Before the journey, it was difficult for him to fall asleep because of excitement.