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Questions & Answers about Под потолком лампа.
Why is the word for "ceiling" (потолок) changed to потолком here?
It's in the Instrumental case. When the preposition под (under) is used to describe a fixed location (where something is), it requires the following noun to be in the Instrumental case. Since потолок is a masculine noun ending in a consonant, it takes the ending -ом.
Wait, the dictionary word is потолок. Where did the last о go in потолком?
Потолок has a "fleeting vowel." In many Russian words, the last vowel (usually о or е) drops out when you add a case ending. So, потолок drops the о to become the stem потолк-, and then we add the Instrumental ending -ом.
Why doesn't the sentence say Под потолком есть лампа?
In Russian, the present tense of the verb "to be" (есть) is often skipped when simply pointing out where things are located in a scene. The word order alone does the job of communicating "There is..."
Why does the sentence start with the location (Под потолком) instead of the subject (лампа)?
In Russian, the most important new information usually goes at the end of the sentence. By putting Под потолком first and лампа last, it translates to "There is a lamp under the ceiling." If you swapped it to Лампа под потолком, it would mean "The lamp is under the ceiling."