Questions & Answers about Гость слушал из вежливости, но уснул от скуки.
Why does this sentence use из for politeness but от for boredom?
This perfectly shows the difference between conscious motives and involuntary reactions! The guest chose to listen out of a sense of character or social duty, which is a conscious motive, so we use из (из вежливости). But falling asleep wasn't a choice; it was an involuntary physical reaction to being bored, so we use от (от скуки).
Both вежливости and скуки are in the Genitive case. Why do they both end in -и?
They belong to different noun groups but happen to share the same ending here. The word вежливость (politeness) is a feminine noun ending in a soft sign, and these always take -и in the Genitive. The word скука (boredom) is a standard feminine noun ending in -а, which usually changes to -ы in the Genitive. However, because of the spelling rule (never write -ы after к, г, х, ж, ч, ш, щ), it takes -и instead.
Why use уснул instead of спал for "fell asleep"?
The verb спал is the imperfective form meaning "was sleeping" (an ongoing state). Here, we need the perfective verb уснул (from уснуть) to show the specific, completed action of transitioning from awake to asleep: he "fell asleep" at a sudden moment.