Malgraŭ la pluvo ni marŝis al la akvofalo.

Because malgraŭ is a preposition meaning despite / in spite of, and it introduces the circumstance being contrasted with the main action.

So the sentence is structured like:

Malgraŭ la pluvo = despite the rain
ni marŝis al la akvofalo = we walked to the waterfall

You could also move that phrase:

Ni marŝis al la akvofalo malgraŭ la pluvo.

That would still be correct. Putting malgraŭ la pluvo first gives it a little more emphasis.

La is used because this is understood as a specific rain situation—the rain that was happening at that time.

So la pluvo here means something like the rain / the rainy weather we were dealing with.

Without la, pluvo would sound more general or less tied to a specific situation. In this sentence, la pluvo is the most natural choice.

Because malgraŭ is a preposition, and its noun normally stays in the regular form:

malgraŭ la pluvo
not malgraŭ la pluvon

In Esperanto, the -n ending is mainly used for:

• direct objects
• direction in certain cases

But after a normal preposition like malgraŭ, you usually do not add -n.

No. In Esperanto, verbs do not change according to the subject.

So:

• mi marŝis = I walked
• vi marŝis = you walked
• ni marŝis = we walked
• ili marŝis = they walked

The ending -is only tells you that the action is in the past. It does not tell you who did it. That is why ni is needed.

The ending -is marks the past tense.

So:

• marŝas = walks / is walking
• marŝis = walked
• marŝos = will walk

The verb stem is marŝ-, and -is tells you the action happened in the past.

Because al already shows direction: to / toward.

So:

ni marŝis al la akvofalo = we walked to the waterfall

The preposition al makes the direction clear, so no extra -n is needed.

Esperanto sometimes uses -n to show direction after certain expressions, but with al, that would be unnecessary. Al already does that job.

It is a compound word:

• akvo = water
• falo = fall

Together, akvofalo literally means water-fall, which matches English waterfall very closely.

This is very common in Esperanto: smaller words are combined to make a clear compound word.

For the same basic reason as la pluvo: it refers to a specific waterfall, not just any waterfall in general.

al la akvofalo = to the waterfall

This suggests that the speaker and listener either already know which waterfall is meant, or the context makes it clear.

No, Esperanto word order is fairly flexible.

This sentence could also be:

Ni marŝis al la akvofalo malgraŭ la pluvo.

That still means the same thing. The original version puts more attention on the contrast: despite the rain.

So the order can change, but the endings and prepositions keep the meaning clear.

In Esperanto, the stress is always on the second-to-last syllable.

So here you get:

• mal-GRAŬ
• la PLU-vo
• ni MAR-ŝis
• al la ak-vo-FA-lo

That regular stress pattern is one of the nice things about Esperanto: once you know the rule, pronunciation is much more predictable than in English.