Spectator delphinum supra euripum videt.

Because spectator is in the nominative singular, the case normally used for the subject of a finite verb.

You can also confirm it from the verb: videt means he/she/it sees, so it needs a singular subject.
So spectator ... videt = the spectator sees.

In Latin, the subject is identified mainly by case ending, not just by position in the sentence.

Because it is the direct object of videt.

The dictionary form is delphinus = dolphin, but when that noun is the direct object, it changes to the accusative singular:

• nominative: delphinus
• accusative: delphinum

So spectator delphinum videt means the spectator sees the dolphin.

Both delphinum and euripum are accusative singular, but they do different jobs.

• delphinum is the direct object of videt
• euripum is the object of the preposition supra

So:

• What does the spectator see? → delphinum
• Above what? → euripum

This is a very common thing in Latin: the same case can be used for more than one reason.

Here supra takes the accusative.

So supra euripum means above the channel/canal/strait depending on the meaning already supplied to the learner.

This whole phrase is a prepositional phrase describing location or position.

Videt is:

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

Its dictionary verb is videre = to see.

The ending -t tells you it means he/she/it sees.

Because that is very common in Latin.

Latin often prefers a pattern like:

• subject
• other information
• verb

So Spectator delphinum supra euripum videt is a very natural Latin order.

But Latin word order is more flexible than English word order, because the endings already show the grammar. The verb does not have to come last, but it often does.

Latin has no articles.

So a noun like spectator can mean:

• a spectator
• the spectator

And delphinum can mean:

• a dolphin
• the dolphin

English has to choose a/an or the, but Latin usually leaves that to context.

Spectator is a third-declension noun.

Its nominative singular is spectator, and its genitive singular is spectatoris.

So in this sentence, spectator is nominative singular and functions as the subject.

A learner might expect all masculine singular subjects to end in -us, but many do not. Third-declension nouns have a wider variety of endings.

Because euripum is already tied to the preposition supra.

Once you see supra, you expect it to have its own object: supra euripum.

That leaves delphinum as the noun directly connected to videt.

So the sentence divides naturally like this:

• Spectator = subject
• delphinum = direct object
• supra euripum = prepositional phrase
• videt = verb

Yes, often they could.

For example, Latin could also say things like:

• Spectator supra euripum delphinum videt
• Delphinum spectator supra euripum videt

The exact emphasis may shift, but the basic meaning stays clear because the endings still show the grammatical roles.

That is one of the big differences from English: Latin depends much more on inflection and much less on fixed word order.

Strictly speaking, Latin often does not need an expressed subject pronoun, because the verb ending already tells you the person and number.

So videt by itself could mean he sees, she sees, or it sees, depending on context.

But a noun like spectator is added when Latin wants to say exactly who is doing the action. Here it makes the sentence clear: it is the spectator who sees.