Ni grimpas la monton.

The -n is the accusative ending. In Esperanto, that usually marks the direct object of the verb.

Here, la monton is the thing being climbed, so it gets -n:

• Ni grimpas la monton = We climb / are climbing the mountain.

This is similar to English, where the mountain is the object of climb.

In this sentence, it is best understood as the ordinary direct-object -n.

Esperanto also has a directional use of -n, but that is typically used with place expressions, often after a preposition or with words of location, such as hejmen or sur la monton.

In Ni grimpas la monton, la monton is simply the object of grimpas: it is the mountain that is being climbed.

Because la never changes. It is invariable.

So you say:

• la monto
• la monton
• la montoj
• la montojn

The noun changes, but la stays la.

Because Esperanto does not need a separate helping verb here.

The ending -as already marks the verb as present tense, so grimpas by itself can mean:

• climb
• climbs
• am climbing
• is climbing
• are climbing

English often uses a form of to be plus -ing, but Esperanto usually just uses the main verb with the correct ending.

It can mean either, depending on context.

The Esperanto present tense -as covers both the simple present and the present progressive that English distinguishes between.

So Ni grimpas la monton could mean:

• We climb the mountain
• We are climbing the mountain

Usually the situation tells you which meaning is intended.

Esperanto has a definite article, la, but no indefinite article.

So:

• monto = a mountain or just mountain
• la monto = the mountain

In your sentence, la monton means the speaker has a specific mountain in mind, or the mountain is already known from context.

Because grimpi can directly take the thing being climbed as its object.

So Esperanto can say:

• grimpi la monton
• grimpi arbon
• grimpi ŝtuparon

That works much like English climb the mountain, climb a tree, or climb the stairs.

If you want to add extra detail about direction, Esperanto can do that, but it is not necessary in this basic sentence.

Yes. Because -n marks the object, Esperanto word order is more flexible than English word order.

The most neutral order is:

• Ni grimpas la monton

But something like:

• La monton ni grimpas

is still understandable, because monton has -n, so you know it is the object, not the subject.

Even so, subject + verb + object is the most normal order for a basic sentence.

Usually, yes.

Esperanto verbs do not change according to person:

• mi grimpas
• vi grimpas
• li grimpas
• ni grimpas

Because grimpas alone does not tell you who is climbing, the subject pronoun is normally included unless the context already makes it obvious.

Yes. Ni can mean either:

• we, including you
• we, not including you

Esperanto does not normally make that distinction in the pronoun itself. Context tells you which meaning is intended.

Like this:

• ni = we
• grimp-as = root grimp-
  • present-tense ending -as
• la = the
• mont-o-n = root mont-
  • noun ending -o
    • accusative ending -n

So the sentence is a good example of how Esperanto builds meaning very regularly from small parts.