Breakdown of Почему он берёт ключ от чужой квартиры?
он
he / it (for a masculine noun) / it (for a neuter noun)
квартира
apartment
ключ
key
почему
why
Questions & Answers about Почему он берёт ключ от чужой квартиры?
Why does the Russian sentence use от (from) when the English translation says "key to"?
In Russian, you don't say the key "to" a lock or door; you say the key "from" it (ключ от). The preposition от always requires the following nouns and adjectives to be in the Genitive case, which is why we see чужой квартиры next.
How is the adjective чужой (someone else's) changing here?
It is modifying квартира (apartment), which is a feminine noun. Because of the preposition от, the phrase must be in the Genitive case (квартиры). Feminine adjectives and pronouns in the Genitive case usually take the ending -ой or -ей. The dictionary (masculine Nominative) form of this word is чужой, and by coincidence, its feminine Genitive form looks exactly the same: чужой.
Why doesn't ключ (key) change its ending? Isn't it the direct object being taken?
Yes, ключ is the direct object, so it is in the Accusative case. However, because ключ is a masculine inanimate noun (an object, not a living person or animal), its Accusative form looks exactly like its basic dictionary form.
I don't recognize the verb берёт. What is its dictionary form?