Cometes supra horizontem apparet.

A simple breakdown is:

• cometes — the subject, nominative singular
• supra — a preposition, meaning above
• horizontem — the object of the preposition, accusative singular
• apparet — the verb, appears / comes into view

So the structure is basically:

subject + prepositional phrase + verb

That is a very normal Latin sentence pattern.

Because Latin has no articles. It does not have separate words for the or a/an.

So:

• cometes can mean a comet or the comet
• horizontem can mean a horizon or the horizon

English has to choose one. In this sentence, the comet and the horizon sound most natural.

Because cometes is a Greek loanword in Latin. Some Greek nouns keep a Greek-looking nominative singular ending in -ēs.

So here cometes is singular, not plural. It is the nominative singular form meaning comet.

That can look strange at first if you are used to Latin nouns where -es often suggests a plural form, but here it does not.

It is usually treated as masculine in Latin.

That may feel surprising, because many first-declension nouns are feminine, but Greek loanwords are a common exception. So a dictionary may give it as something like cometes, -ae, m.

In this sentence, you do not actually see the gender affecting anything, because there is no adjective agreeing with it.

Because supra normally takes the accusative when used as a preposition.

So:

• supra = above
• horizontem = accusative singular of horizon

Together, supra horizontem means above the horizon.

This is one of those places where English and Latin do not match neatly. In English, above feels like a simple location word, but in Latin the preposition supra still regularly governs the accusative.

Apparet is:

• present tense
• active voice
• indicative mood
• third person singular

It comes from appareo / apparere, meaning to appear, to become visible, or to be evident.

It is singular because the subject cometes is singular.

Because cometes is singular.

In Latin:

• apparet = he/she/it appears
• apparent = they appear

Since there is only one comet, the verb must be third person singular, so apparet is correct.

Here it most naturally means comes into view or is visible.

So the idea is that the comet is appearing above the horizon in a visual sense.

The same verb can sometimes mean something like seems or is evident, but in this sentence the astronomical/visual meaning is clearly the intended one.

No. Latin word order is fairly flexible because the endings show the grammatical roles.

This sentence is perfectly natural as written:

Cometes supra horizontem apparet.

But other orders are also possible, for example:

• Supra horizontem cometes apparet.
• Apparet cometes supra horizontem.

Those versions would shift the emphasis a bit, but the basic meaning would stay the same.

The original order is a nice straightforward one, and placing the verb at the end is very common in Latin.

A reconstructed classical pronunciation would be approximately:

ko-MEH-tehs SOO-pra ho-ri-ZON-tem ap-PAH-ret

A few helpful points:

• c is always hard, like k
• u sounds like oo
• r is ideally tapped or trilled
• h is lightly pronounced
• stress falls on:
  • cometes → co-ME-tes
  • supra → SU-pra
  • horizontem → ho-ri-ZON-tem
  • apparet → ap-PA-ret

If you want the form with macrons, it is often written:

Comētēs suprā horizōntem appāret.