Breakdown of На новой кровати лежит тяжёлый матрас, а рядом — чистая наволочка и новый пододеяльник.
Questions & Answers about На новой кровати лежит тяжёлый матрас, а рядом — чистая наволочка и новый пододеяльник.
Because На новой кровати is a location phrase (meaning on the new bed), and Russian often puts location first to set the scene. The “real” grammatical subject of the first part is тяжёлый матрас (the thing that is lying there).
на + Prepositional is used for surfaces and “on top of” meanings: на кровати = on the bed.
в + Prepositional usually means “inside” something: в коробке (in a box), в комнате (in a room). A bed is treated as a surface for this meaning.
It’s Prepositional case after на (location use): на чём? → на кровати.
кровать is feminine, and in the prepositional singular it becomes кровати; the adjective agrees: новой (fem., prep. sing.).