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.

In Esperanto, the verb already includes the idea of tense, so no separate helping verb like English am / is / are is needed for ordinary present-tense verbs.

• Ni grimpas = We climb or We are climbing
• Ni descendas = We descend or We are descending

So -as by itself already carries the present-tense meaning.

They are opposite movements:

• grimpi = to climb, to go upward with effort
• descendi = to descend, to go downward

In this sentence, they describe movement along the same slope, alternating between upward and downward stretches.

A learner should also notice that grimpi is not exactly the same as just iri supren in every context. Grimpi often suggests climbing, scrambling, or making your way upward, not merely moving upward in a simple neutral way.

Laŭ means along, according to, or in accordance with, depending on context. Here it means along.

So laŭ la deklivo means along the slope.

It tells you the path or direction of movement: the climbing and descending happen following the line of the slope.

The article la means the.

Esperanto uses la when the thing is specific or understood from the situation. In this sentence, the speaker has a particular slope in mind, so la deklivo means the slope.

If you said just laŭ deklivo, that would sound different and less natural here. The sentence is talking about a definite slope, not just any slope in general.

Deklivo means slope, incline, or sloping surface.

It comes from the idea of something leaning or slanting. In the sentence, it refers to the terrain or surface along which the movement happens.

So laŭ la deklivo gives the image of moving along a hillside or some other inclined area.

Because these verbs are being used intransitively here: they describe movement, not action on a direct object.

• Ni grimpas = we climb
• Ni descendas = we descend

There is no direct object receiving the action, so there is no -n ending on a noun.

Instead, the sentence adds a prepositional phrase:

• laŭ la deklivo

That tells where/how the movement happens, not what is being acted on.

Usually, no, not in normal standard Esperanto.

Esperanto verbs do not change form according to person, so grimpas could mean:

• I climb
• you climb
• he/she/it climbs
• we climb
• they climb

Because of that, the subject pronoun is normally stated unless the subject is already completely obvious from context. So Ni is helpful and standard here.

The comma helps separate the two alternating parts of the jen ... jen ... construction:

• jen grimpas
• jen descendas

It makes the rhythm clearer and the alternation easier to read. In practice, punctuation with this kind of structure is often used to show the pause between the two contrasting or alternating actions.

They can be similar, but they are not exactly the same in feel.

• foje ... foje ... means sometimes ... sometimes ...
• jen ... jen ... is more vivid and immediate, like now ... now ... or first ... then ...

So in this sentence, jen ... jen ... makes the movement feel more alive and alternating right before your eyes. It is especially good for descriptive narration.