«Мальчик идёт под мостом» - «он проходит под лестницей». Why instrumental is used since movement is implied? I thought that instumental is used when the subject of the sentence does not change its position (like the prepositional case with «в» or «на»). Does «он проходит под лестницу» have different meaning?
The question arose when I watched a video about prepositions of direction. In the video there where the above examples which were translated as "the boy is walking under the bridge" and "he walks under the ladder". In my two grammar books it is written that when "под" denotes movement to a position underneath something, accusative is used "он сели под дерево - they sat down under the tree". On the contrary, instrumental is used when location is implied "стоять под мостом - to stand under the bridge". Furthermore in the same article of this book,in another example with "под+instrumental" movement is denoted "плавать под водой - to swim under water". Is there a difference of meaning between "под+instrumental" and "под+accusative" when movent is implied in the phrase? Is there a possibility that accusative denotes movement until the place and staying there and instumental passing through under the place and walking away afterwards?