Breakdown of Он не знает, безопасно ли спускаться в пещеру.
не
not
он
he / it (for a masculine noun) / it (for a neuter noun)
знать
to know
в
in / to (direction)
Questions & Answers about Он не знает, безопасно ли спускаться в пещеру.
Does спускаться use the same с- prefix for 'descent' that we see in verbs of motion like сходить?
Yes! The prefix с- often adds the meaning of 'down' or 'off a surface.' While сходить means to go down by walking (like stepping down stairs), спускаться (from пускать, 'to let') literally means 'to lower oneself.' It is the standard verb for descending into a cave, a basement, or down a mountain.
How does the word ли work in this sentence?
The particle ли translates to 'whether' or 'if' for indirect questions. In Russian, ли is never the first word in its clause; it must come right after the word being questioned. Because the main doubt here is whether the action is safe, ли follows directly after безопасно.
Why is the imperfective verb спускаться used here instead of the perfective спуститься?
Both can work, but they emphasize different things. The imperfective спускаться focuses on the process: is the actual journey down safe (e.g., are the ropes secure)? The perfective спуститься would focus on the result: is it safe to successfully reach the bottom?
Why is it в пещеру (accusative) rather than в пещере (prepositional)?