Questions & Answers about Ni jen grimpas, jen descendas laŭ la deklivo.
Jen ... jen ... is a very common Esperanto pattern meaning something like now ... now ..., at one moment ... at another ..., or sometimes ... sometimes ... depending on context.
So in this sentence, it shows alternation:
- jen grimpas
- jen descendas
That means the action keeps changing back and forth: first climbing, then descending.
This use of jen often presents something vividly, as if you are watching it happen.
The ending -as marks the present tense in Esperanto.
So:
- grimpas = climbs / are climbing
- descendas = descends / are descending
Esperanto often uses the present tense for actions that are happening in front of you, are generally true in the current situation, or are being described vividly. In this sentence, it gives the sense of ongoing movement along the slope.