Tonitrus longus erat, sed fulgur procul supra montes breve visum est.

Because Latin adjectives must agree with the nouns they describe in gender, number, and case.

• tonitrus is masculine singular nominative, so its adjective is longus
• fulgur is neuter singular nominative, so its adjective is breve

So the difference is not about meaning, but about grammatical agreement.

Both are nominative singular, because each is the subject of its clause:

• Tonitrus longus erat = the thunder was long
• fulgur ... visum est = the lightning was seen / appeared

In Latin, the subject of a finite verb is normally in the nominative.

Tonitrus is a 4th-declension masculine noun meaning thunder.

A learner may expect a noun ending in -us to be 2nd declension, but tonitrus is not. Its genitive is tonitrūs.

So in this sentence:

• tonitrus = nominative singular
• longus agrees with it

Fulgur is a 3rd-declension neuter noun meaning lightning or a flash of lightning.

Important things to notice:

• it is neuter
• its nominative singular is fulgur
• its genitive singular is fulguris

Because it is neuter, the adjective is breve, not brevis.

Erat is the imperfect of esse, while fuit is the perfect.

The imperfect often describes a situation as ongoing, continuous, or part of the background. That fits well with tonitrus longus erat, because the thunder is being described as lasting for some time.

So erat gives a sense like:

• was
• was continuing
• lasted / was prolonged in description

A learner will often notice that Latin uses the imperfect where English simply uses was.

Visum est is the perfect passive of videre.

Literally, it means was seen. In context, it can also be understood more naturally as appeared or was visible.

Formally:

• visum = perfect passive participle, neuter singular
• est = is, used here to make the perfect passive

Because the subject is fulgur (neuter singular), the participle is also visum.

In a perfect passive form like visum est, the participle agrees with the subject.

Here the subject is fulgur, which is:

• neuter
• singular
• nominative

So Latin uses visum, the neuter singular form.

If the subject were feminine singular, you would expect visa est. If masculine singular, visus est.

It can suggest either, depending on context.

• very literally: was seen
• more idiomatically in many contexts: appeared

Since breve is describing how the lightning presented itself, English may naturally render this as something like appeared brief.

So a learner should recognize that Latin passive forms of videre can sometimes sound more natural in English as appear rather than a stiff was seen.

Latin word order is much freer than English word order.

Breve still belongs with fulgur, because agreement shows the connection. Its placement before visum est also helps the phrase read as a unit: breve visum est = appeared brief / was seen as brief.

So the meaning is not decided mainly by position, but by:

• agreement
• case
• verb form
• overall clause structure

Procul is an adverb meaning far away, in the distance, or from afar.

Here it helps describe where the lightning was seen: it was visible far off, not nearby.

It modifies the whole idea of the lightning’s appearance/location, not a single noun in the way an adjective would.

Montes is accusative plural.

It is accusative because it follows the preposition supra, which normally takes the accusative and means above, over, or beyond.

So:

• supra = above/over
• montes = mountains, accusative plural

Together: above the mountains

Because Classical Latin has no articles.

So:

• tonitrus can mean thunder or the thunder
• fulgur can mean lightning, the lightning, or a flash of lightning
• montes can mean mountains or the mountains

English has to choose whether to say the, a, or nothing at all, but Latin leaves that to context.

Yes. Longus here refers to duration, not physical length.

Latin often uses adjectives like:

• longus = long, prolonged
• brevis / breve = short, brief

So tonitrus longus means the thunder lasted a long time, and fulgur breve means the lightning was brief in duration or appearance.

That is a very normal Latin way to describe these two events.