Poste ni descendas en la valon.

Because poste is an adverb, meaning afterward / afterwards / then / later.

• poste = afterwards, then
• post = after as a preposition, so it must be followed by something

So:

• Poste ni descendas en la valon. = Afterwards/Then we descend into the valley.
• Post la manĝo ni descendas en la valon. = After the meal we descend into the valley.

In your sentence, there is no noun after post-, so the adverb poste is the right form.

Yes. Esperanto word order is fairly flexible, especially with adverbs.

These are all possible, with slightly different emphasis:

• Poste ni descendas en la valon.
• Ni poste descendas en la valon.
• Ni descendas poste en la valon.

The first version is very natural if you are telling a sequence of events: first this happens, then that happens.

So putting poste at the beginning often gives a nice narrative feel.

Ni is the Esperanto subject pronoun meaning we.

Common personal pronouns include:

• mi = I
• vi = you
• li = he
• ŝi = she
• ĝi = it
• ni = we
• ili = they

Here ni is the subject of descendas, so it means we descend / we go down.

Because -as is the Esperanto ending for the present tense.

So:

• descendas = descend / are descending / go down
• descendis = descended
• descendos = will descend
• descendus = would descend
• descendi = to descend

In Esperanto, -as does not specifically mean only right now. It can also be used for:

• habitual action
• general statements
• narrative present

So ni descendas can mean something like:

• we descend
• we are descending
• we go down

depending on context.

The base form is descendi, meaning to descend.

You can think of descendas as:

• descend- = the root
• -as = present-tense verb ending

So:

• descendi = to descend
• descendas = descends / are descending / go down

This verb already includes the idea of going downward, so Esperanto does not need a separate word for down here.

Because the -n on valon shows direction toward a place.

This is a very important Esperanto pattern:

• en la valo = in the valley (location, no movement into it)
• en la valon = into the valley (movement toward/into it)

So in your sentence:

• ni descendas en la valon = we descend into the valley

The -n here is not a direct object in the English sense. It is the accusative of direction, showing movement toward the inside of the valley.

Because en emphasizes movement into the place, while al means movement to/toward it.

Compare:

• en la valon = into the valley
• al la valo / al la valon = to the valley / toward the valley

In this sentence, the idea is that you end up inside the valley, not just heading toward it. So en la valon is the most natural choice.

La is the definite article, meaning the.

So:

• valo = a valley / valley
• la valo = the valley

Esperanto has only one article, la, and it does not change for gender, number, or case. Even when the noun takes -n, the article stays the same:

• la valo
• la valon

The sentence uses la because it refers to a particular valley, probably one already known from the context.

Roughly as des-TSEN-das, with the stress on the second-to-last syllable.

A few useful pronunciation points:

• c in Esperanto is always like ts in cats
• sc is pronounced as s + ts
• stress is always on the penultimate syllable

So descendas is pronounced approximately:

des-TSEN-das

not like English descends.

Yes, essentially. Descendi means to descend, to go down, or to move downward.

So depending on style, ni descendas may sound in English like:

• we descend
• we go down
• we head down

Esperanto often uses a single verb where English might choose a more everyday phrasal expression.