Ni ĉeestas la prelegon en la observatorio.

Because ĉeesti is a transitive verb in Esperanto. It takes a direct object.

• ĉeesti = to attend, to be present at
• So the thing being attended gets -n
• la prelegon = the lecture as the direct object

That is why Esperanto says:

• Ni ĉeestas la prelegon. = We are attending the lecture.

Even though English uses attend without a special ending, Esperanto shows the object clearly with -n.

Ĉeestas comes from ĉeesti.

It means:

• to attend
• to be present at
• to be in attendance at

So Ni ĉeestas la prelegon means that we are present at the lecture / we are attending the lecture.

A useful point: Esperanto uses ĉeesti as a single verb, where English often uses a verb plus a preposition, such as be present at.

Because ĉeesti already includes that idea.

In English, we say:

• be present at the lecture

But in Esperanto, the verb itself works directly with an object:

• ĉeesti la prelegon

So you do not normally say:

• ĉeesti ĉe la prelego for this meaning

That would sound wrong or unnecessary in standard usage.

Because en la observatorio shows location, not motion toward a place.

• en la observatorio = in the observatory
• no -n, because it tells us where the lecture attendance is happening

You would use -n after en only when there is movement into something:

• Ni iras en la observatorion. = We are going into the observatory.

But here nobody is moving. The sentence just says where the event is taking place.

• ni = we
• -as is the Esperanto ending for the present tense

So ĉeestas means attend / are attending / are present at in the present.

Esperanto verbs do not change depending on the subject:

• mi ĉeestas = I attend
• vi ĉeestas = you attend
• li ĉeestas = he attends
• ni ĉeestas = we attend

The verb ending stays -as for all persons in the present tense.

They are related, but not exactly the same.

• ĉeesti la prelegon = to attend the lecture
• esti ĉe la prelego = to be at the lecture

Very often they are close in meaning, but ĉeesti is the more natural verb for attend.

Compare:

• Ni ĉeestas la prelegon. = We are attending the lecture.
• Ni estas ĉe la prelego. = We are at the lecture.

The second one focuses more on location/presence; the first one is the standard verb for taking part as attendees.

Not exactly. Prelego usually means a lecture, talk, or formal presentation.

It is closer to:

• a lecture at a university
• a public talk
• a formal speech on a topic

It is not usually the best word for every kind of class or lesson.

Related words:

• leciono = lesson
• kurso = course
• prelego = lecture / talk

So la prelegon suggests a lecture rather than, for example, a small interactive class.

La is Esperanto’s definite article, meaning the.

In this sentence:

• la prelegon = the lecture
• la observatorio = the observatory

It is used because the speaker has some specific lecture and some specific observatory in mind.

Esperanto has only one definite article: la. It does not change for gender, number, or case.

So you always use the same form:

• la prelego
• la prelegoj
• la prelegon
• la observatorio

A helpful approximate pronunciation is:

• ĉe sounds roughly like che in check
• e is like e in bet, but pure and steady
• stress in Esperanto falls on the second-to-last syllable

So ĉeestas is pronounced approximately:

• che-eh-STAS

Syllable breakdown:

• ĉe-es-tas

The stress is on es.

Also, ĉ is always pronounced like ch in church.

Yes. Esperanto word order is fairly flexible because grammar endings show the relationships clearly.

The neutral order here is:

• Ni ĉeestas la prelegon en la observatorio.

But you could also say:

• En la observatorio ni ĉeestas la prelegon.
• La prelegon ni ĉeestas en la observatorio.

These are grammatically possible, though they may shift emphasis.

The most natural version for ordinary use is usually the original one.

In practice, it describes the whole situation: the attending is happening in the observatory, and by implication the lecture is there too.

So the sentence naturally means:

• We are attending the lecture in the observatory.

English has a similar structure, where the location phrase can apply to the event as a whole.

If context mattered, Esperanto could rephrase things more explicitly, but this sentence is perfectly normal as it stands.

Yes. That is a very close translation.

Depending on context, ĉeesti can be translated as:

• attend
• be present at
• be in attendance at

So all of these are possible ways to express the idea:

• We are attending the lecture in the observatory.
• We are present at the lecture in the observatory.
• We are in attendance at the lecture in the observatory.

Attend is usually the most natural everyday English translation.

Yes. Observatorio is a normal noun meaning observatory.

Like most Esperanto nouns, it ends in -o:

• observatorio = observatory
• observatorion = observatory as a direct object or destination
• observatorioj = observatories

It is an international-style word, so it looks familiar to many English speakers.