Mi vidas du aferojn, nome rokon kaj akvofalon.

Because aferojn is a plural direct object.

In Esperanto:

• -o = noun
• -j = plural
• -n = accusative, usually marking the direct object

So:

• afero = a thing
• aferoj = things
• aferojn = things, as the direct object

In Mi vidas du aferojn, the verb vidas means see, and du aferojn is what is being seen.

They also take -n because they are in apposition to du aferojn. In other words, they are naming what those two things are.

So the structure is:

• du aferojn = two things
• nome rokon kaj akvofalon = namely, a rock and a waterfall

Since rokon and akvofalon refer back to the direct object, they match its grammatical role and also take the accusative ending.

This is very natural in Esperanto.

Because each one names one item:

• rokon = a rock
• akvofalon = a waterfall

Together they make up the two things. Esperanto does not make them plural just because the total is two. Compare it to English: two things, namely a rock and a waterfall. Each listed item is singular.

Nome means namely, to wit, or specifically.

It introduces a more exact explanation of what was just mentioned. Here:

• du aferojn = two things
• nome rokon kaj akvofalon = namely, a rock and a waterfall

So nome tells you that the speaker is now specifying exactly which two things they mean.

Esperanto has no indefinite article. So where English says:

• a rock
• a waterfall

Esperanto simply says:

• roko
• akvofalo

When needed in the sentence, they take their grammatical endings, so here they become:

• rokon
• akvofalon

Esperanto does have a definite article, la, for the, but there is no separate word for a/an.

The comma helps mark nome rokon kaj akvofalon as an explanatory addition. It is similar to how English often uses a comma before namely.

So the comma signals:

• first, a general statement: I see two things
• then, a clarification: namely, a rock and a waterfall

This punctuation is very natural and helpful for readability.

Normally, yes. Vidi is a transitive verb, meaning it takes a direct object. The thing seen gets -n.

Examples:

• Mi vidas la domon. = I see the house.
• Ŝi vidas birdojn. = She sees birds.
• Mi vidas du aferojn. = I see two things.

So in this sentence, the accusative is exactly what you would expect after vidas.

Yes. Akvofalo is a compound noun:

• akvo = water
• falo = fall

Together:

• akvofalo = waterfall

Then the sentence adds -n because it is in the accusative:

• akvofalon

Compound words are very common in Esperanto, and they are often quite straightforward like this.

Yes, in many contexts you could, but the nuance is a little different.

• afero often means thing, matter, or item in a broad, everyday sense.
• aĵo often means a concrete thing/object, especially when formed from another root.

In this sentence, aferojn sounds perfectly normal for two things. Depending on context, aĵojn might also work, but aferojn is a very natural choice.

Esperanto word order is fairly flexible, but this version is the most straightforward and natural.

Mi vidas du aferojn, nome rokon kaj akvofalon.

This order works well because it gives:

1. the action: Mi vidas
2. the general object: du aferojn
3. the clarification: nome rokon kaj akvofalon

You might rearrange parts in special contexts, but for a learner, this sentence is a good standard model.

Because Esperanto numbers like du, tri, kvar, and so on do not change for case or number.

So:

• du aferoj = two things
• du aferojn = two things, as a direct object

The noun changes, not the number. This is different from some languages where numerals may agree more fully, but in Esperanto the numeral itself stays the same.