Spiritus aegri brevis est.

Spiritus is a fourth-declension noun, not a second-declension one.

A lot of English speakers first learn that masculine nouns in -us are often second declension, but -us can also belong to the fourth declension. The difference shows up in the other forms:

• second declension: servus, servī
• fourth declension: spiritus, spiritūs

So the ending -us by itself does not guarantee second declension.

Here spiritus is nominative singular, because it is the subject of est.

A good way to see that is this:

• est = is → singular verb
• brevis = short → predicate adjective describing the subject
• so the thing that is short must be a singular subject

Therefore spiritus is the nominative singular subject: the breath/spirit.

Because it means of the sick person.

Latin often uses the genitive where English uses of or an apostrophe:

• spiritus aegri = the breath of the sick person
• or in natural English, the sick person's breath

So aegri is not describing spiritus directly as an adjective in the same case. Instead, it shows a relationship: whose breath it is.

It is formally an adjective, from aeger, aegra, aegrum (sick), but here it is being used substantively—that is, like a noun.

So aegri really means something like:

• of the sick man
• of the sick person
• of the sick one

Latin does this very often. An adjective can stand on its own when the noun is understood from context.

Here brevis is nominative singular, agreeing with spiritus.

It is a third-declension adjective. In the nominative singular:

• masculine: brevis
• feminine: brevis
• neuter: breve

Since spiritus is masculine, the correct form is brevis, not breve.

Also, because this is a sentence with est, brevis is a predicate adjective: it says what the subject is.

Because that would mean something different.

• spiritus aegri brevis est = the sick person's breath is short
• spiritus aeger brevis est would make aeger agree with spiritus, so it would mean something like the sick breath is short or the breath, being sick, is short

In other words:

• aegri = of the sick person
• aeger = sick describing the noun directly

So the genitive is necessary for the meaning here.

Yes, in isolation it could.

Without context, aegri can be:

• genitive singular = of the sick man/person
• nominative plural masculine = the sick men/persons

But in this sentence, the grammar shows that it must be genitive singular:

• est is singular
• brevis is singular
• the sentence works as spiritus aegri = the breath of the sick person

So context resolves the ambiguity.

Because Latin word order is much freer than English word order.

English depends heavily on word order for meaning. Latin depends much more on inflected endings. So Latin can move words around for style, emphasis, or rhythm.

This order is very natural:

• spiritus = subject
• aegri = dependent genitive attached to it
• brevis = predicate adjective
• est = verb, often placed near the end

So the sentence is structured clearly even though it does not match normal English order.

Macrons can help distinguish forms that are otherwise spelled the same.

For example, with spiritus:

• spiritus = nominative singular
• spiritūs = genitive singular or nominative plural

Without macrons, those look identical in ordinary writing.

With aegri, however, even macrons do not solve everything:

• aegrī can still be either genitive singular or nominative plural

So macrons are helpful, but you still need syntax and context to parse the sentence correctly.