Ni grimpis sur grandan rokon.

The ending -is marks the past tense in Esperanto.

So:

• grimpi = to climb
• grimpas = climbs / is climbing
• grimpis = climbed
• grimpos = will climb

So Ni grimpis means we climbed.

Sur means on, onto, or on top of, depending on context.

In this sentence, it shows the destination of the climbing: they climbed onto a big rock.

So sur grandan rokon means something like onto a big rock rather than just to a rock in a general sense.

There are two things happening here:

1. rokon has -n because the phrase shows movement toward a place after sur.
2. grandan also gets -n because adjectives must agree with the nouns they describe.

So:

• roko = rock
• rokon = rock with accusative -n
• granda roko = a big rock
• grandan rokon = a big rock, with both words marked to match

This agreement is very normal in Esperanto.

This is an important Esperanto distinction:

• sur granda roko = on a big rock; location, no movement onto it is being emphasized
• sur grandan rokon = onto a big rock; movement toward that place

So in your sentence, sur grandan rokon is used because the action involves going onto the rock.

A similar contrast is:

• La kato estas sur la tablo = the cat is on the table
• La kato saltis sur la tablon = the cat jumped onto the table

Al means to or toward, but it does not tell you the kind of destination.

• al roko = to a rock
• sur rokon = onto a rock

Since the sentence specifically means climbing onto the top or surface of the rock, sur is the better choice.

Esperanto has no indefinite article. English uses a or an, but Esperanto simply does not.

So:

• roko can mean a rock or just rock, depending on context
• la roko means the rock

That is why grandan rokon naturally means a big rock without a separate word for a.

Not exactly.

• roko usually means a rock, especially a larger rock, rock mass, or rocky formation
• ŝtono usually means a stone, often smaller or more like a piece of rock

So grandan rokon suggests a fairly substantial rock, something big enough to climb onto.

Not completely. Esperanto word order is fairly flexible because endings show grammatical roles.

The normal order here is:

• Ni = subject
• grimpis = verb
• sur grandan rokon = prepositional phrase

But other orders are possible for emphasis, such as:

• Sur grandan rokon ni grimpis

That still means the same thing. The original order is just the most neutral and natural.

Normally, Esperanto does include the subject pronoun.

So you say:

• Mi grimpis = I climbed
• Ni grimpis = we climbed

Unlike some languages, Esperanto verbs do not change enough to make the subject obvious by themselves, so the pronoun is usually needed unless the context is very clear.

A simple approximate pronunciation is:

nee GREEM-pees soor grahn-DAHN ROH-kon

A few helpful points:

• i sounds like ee
• a sounds like ah
• o sounds like oh
• stress is usually on the second-to-last syllable

So:

• Ni → nee
• grimpis → GREEM-pis
• grandan → grahn-DAHN
• rokon → ROH-kon

Esperanto pronunciation is very regular, so once you learn the sound system, sentences like this become easy to read aloud.