Ni vidas eklipson de la observatorio.

Because eklipson is the direct object of vidas.

In Esperanto, the thing directly affected by the verb usually takes the accusative ending -n. Here:

• Ni = we
• vidas = see
• eklipson = an eclipse

So the eclipse is what we see, and that is why it gets -n.

The dictionary form is eklipso. In the sentence, it becomes eklipson.

Esperanto uses la only for a definite noun, meaning a specific one already known or clearly identified.

So:

• eklipson = an eclipse
• la eklipson = the eclipse

If the intended meaning is just an eclipse, then leaving out la is correct.

If you were talking about one specific eclipse that both speaker and listener know about, you would say la eklipson.

Here it most naturally means from the observatory or at the observatory, depending on the intended meaning already given.

The preposition de is very broad in Esperanto and can mean things like:

• of
• from
• sometimes a relation like coming from or connected with

So in this sentence, de la observatorio gives the source or viewpoint of the seeing.

However, many learners notice that this is not the only possible preposition here.

Yes, that is a very reasonable question.

Depending on the exact situation, many speakers might prefer:

• el la observatorio = from inside the observatory, out from it
• ĉe la observatorio = at the observatory / by the observatory
• de la observatorio = from the observatory, with a broader or less specific sense

So if the meaning is specifically we are inside the observatory and see an eclipse from there, el la observatorio may sound clearer to many speakers.

If the meaning is simply while at the observatory, then ĉe la observatorio can also be natural.

So de is possible, but it can be less precise than other choices.

Yes.

Because de often connects nouns, eklipso de la observatorio could also be understood as something like:

• the observatory’s eclipse
• an eclipse of the observatory
• some other noun-to-noun relationship

In your sentence, context probably makes the intended meaning clear, but grammatically de can create ambiguity.

If you want to make the location or viewpoint clearer, you could say:

• Ni vidas eklipson el la observatorio.
• De la observatorio ni vidas eklipson.

Those versions make it easier to read the phrase as modifying the whole action of seeing.

Because it is part of a prepositional phrase: de la observatorio.

Normally, a noun after a preposition does not take the accusative -n just because it follows that preposition.

So:

• de la observatorio = from the observatory
• not de la observatorion

The -n here is only on eklipson because that noun is the direct object of vidas.

No. In Esperanto, verb forms do not change for person or number.

So vidas can mean:

• I see
• you see
• he sees
• she sees
• it sees
• we see
• they see

That is why the subject pronoun is usually necessary:

• mi vidas = I see
• vi vidas = you see
• ni vidas = we see

Without ni, the sentence would not tell you who is seeing.

Yes. Esperanto word order is fairly flexible because the accusative -n helps show what is the object.

So these can all work, with slightly different emphasis:

• Ni vidas eklipson de la observatorio.
• De la observatorio ni vidas eklipson.
• Eklipson ni vidas de la observatorio.

The first one is the most neutral.
Putting de la observatorio first gives more emphasis to the location or viewpoint.

It is the present tense, and Esperanto does not normally make a basic grammatical distinction between:

• we see
• we are seeing

Both are often expressed with ni vidas.

Context tells you whether the action is:

• happening right now
• habitual
• general

So ni vidas can correspond to either English form, depending on the situation.

They are:

• ni = we
• vidi = to see
• eklipso = eclipse
• de = of, from
• la = the
• observatorio = observatory

In the sentence:

• vidas is the present-tense form of vidi
• eklipson is eklipso with the accusative ending -n

Adding la before eklipson makes the eclipse definite:

• la eklipson = the eclipse

But there is another issue: with de la observatorio, the phrase may now sound even more like it is attached directly to eklipson, as if it meant:

• the observatory’s eclipse
• the eclipse of the observatory

So if your meaning is really We see the eclipse from the observatory, a clearer version might be:

• Ni vidas la eklipson el la observatorio.
• De la observatorio ni vidas la eklipson.

These reduce the chance of misunderstanding.