Questions & Answers about У тонких стен плохо спать.
Why does тонких end in -их instead of the standard genitive plural ending -ых?
The dictionary form is тонкий (thin). Even though the standard genitive plural ending for adjectives is -ых, Russian spelling rules state we can never write the letter ы after к, г, х, ж, ч, ш, or щ (the 7-letter spelling rule). Because the stem ends in к, we are forced to use -их instead.
Where is the genitive plural ending on the noun стен?
It has a zero ending! The dictionary form is стена (wall). For most feminine nouns ending in -а, you form the genitive plural by simply dropping the -а. Therefore, стена shrinks down to стен.
Doesn't the preposition у usually mean "someone has"? Why does it mean "by" here?
You are right that у is used to express ownership (like У меня есть... - literally "At me there is..."). However, its literal physical meaning is "by", "at", or "near" an object. Whether you are using it for ownership or location, it always requires the following words to be in the genitive case.
There is no word for "it" or "is" in this sentence. How does плохо спать translate to "it is bad to sleep"?