La kometo kaj asteroido estas ĉielkorpoj, ne nebulozoj.

Because la kometo kaj asteroido is a compound subject: it names two things together.

• kometo = one comet
• asteroido = one asteroid
• kometo kaj asteroido = a comet and an asteroid, so together they are plural

So the predicate noun is plural:

• one comet is a ĉielkorpo
• one asteroid is a ĉielkorpo
• both together are ĉielkorpoj

The same logic explains nebulozoj in the negative contrast.

In Esperanto, one la can apply to both nouns joined by kaj.

So:

• la kometo kaj asteroido
• la kometo kaj la asteroido

can both be grammatical, depending on style and emphasis.

Using la only once is very normal when the article clearly belongs to the whole coordinated phrase. English does this too sometimes in structure, though less obviously.

Because nothing here is a direct object.

In Esperanto:

• the subject does not take -n
• a predicate noun after estas also does not take -n

In this sentence:

• La kometo kaj asteroido = subject
• ĉielkorpoj = predicate noun after estas
• nebulozoj = another predicate noun, contrasted with ne

So nominative forms are used throughout.

Because Esperanto verbs do not change according to person or number.

So:

• mi estas = I am
• li estas = he is
• ili estas = they are

The ending -as always marks present tense. Unlike English, Esperanto does not have separate forms like is/am/are in the present tense.

Because here ne is negating only the second label, not the whole sentence.

• estas ĉielkorpoj, ne nebulozoj = are celestial bodies, not nebulae

This is a contrast pattern:

• X estas A, ne B = X is A, not B

If you moved ne before the verb:

• La kometo kaj asteroido ne estas nebulozoj

that would simply mean The comet and asteroid are not nebulae, without explicitly saying what they are instead.

It is a compound word:

• ĉielo = sky, heaven
• korpo = body
• ĉielkorpo = celestial body
• ĉielkorpoj = celestial bodies

In compounds, Esperanto often drops the final -o of the first noun stem:

• ĉielo → ĉiel-
• korpo stays as the main word

Then -j is added for the plural.

The comma marks a contrast, much like in English:

• ..., not nebulae

It helps separate the main statement from the correcting or contrasting phrase.

In Esperanto, punctuation is somewhat style-based in cases like this, but the comma is very natural here because it makes the contrast clearer and easier to read.

Yes, that is also a very natural Esperanto sentence.

The difference is mostly in how the speaker chooses to present the idea:

• Kometoj kaj asteroidoj estas ĉielkorpoj = uses plural class names
• La kometo kaj asteroido estas ĉielkorpoj = can present the nouns more as definite items, or sometimes as representative categories depending on context

So a learner should know that Esperanto can express general truths in more than one way. The exact choice depends on nuance and context, not just grammar.

Yes.

After estas, both ĉielkorpoj and nebulozoj are noun phrases describing what the subject is or is not.

So the structure is:

• La kometo kaj asteroido = subject
• estas = linking verb
• ĉielkorpoj = positive classification
• ne nebulozoj = rejected classification

This is similar to English:

• They are teachers, not students.

Both teachers and students are predicate nouns there, and the Esperanto sentence works the same way.