Questions & Answers about Я положил рубин в сейф.
Why use положил rather than поставил here?
In Russian, placement verbs are very specific. Положить (the perfective form of класть) means to lay something down flat. Since a ruby (рубин) doesn't have a designated base to stand upright on, you lay it down rather than stand it up, which would require the verb поставить.
Doesn't the preposition в usually take the prepositional case? Why is сейф in the accusative?
The preposition в takes the prepositional case for static location (where something currently is), but it takes the accusative case for direction or destination (where something is going). Because you are actively putting the ruby into the safe, it requires the accusative case to show motion toward a destination.
Since рубин is the direct object receiving the action, why doesn't its ending change?
Рубин is indeed in the accusative case here, but because it is a masculine inanimate noun, its accusative form is identical to its nominative form. If you were putting a masculine animate noun into the safe (like a spider, паук), it would take the animate accusative ending and become паука.