Lesson 13. Of Nouns: Fourth Declension

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We now come to the Fourth Declension of nouns. We have already learned how to recognize and decline the First, Second, and Third Declensions, and we have learned to trust one guiding principle above all others: we know a noun’s declension, not from how it looks in the Nominative, but from its Genitive singular ending. That principle remains our safeguard here, because the Fourth Declension can easily be confused with the Second if we look only at the Nominative.

The First Declension is recognized by a Genitive singular ending in -ae, or -es when the Nominative ends in -e. The Second Declension is recognized by a Genitive singular ending in -i. The Third Declension is recognized by a Genitive singular ending in -is. Now we add the Fourth Declension, which is recognized by a Genitive singular ending in -us. If we fix that firmly in mind, the new forms will not trouble us.

It is true that Fourth Declension nouns have only two possible Nominative endings, which is a welcome relief after the variety of the Third Declension. A Fourth Declension noun will have a Nominative ending in -us or in -u—and that is all. Yet we must not jump to conclusions from that simplicity, because the Second Declension also has many nouns ending in -us, such as dominus. Therefore we must always ask: what is the Genitive singular? If the Genitive singular ends in -i, the noun is Second Declension. If the Genitive singular ends in -us, the noun is Fourth Declension. This is the difference between dominus and spiritus. They look similar in the Nominative, but they belong to different declensions because their Genitives differ.

Here we feel more clearly why vocabulary study matters. There is often no sure way to know a noun’s declension simply by looking at the Nominative form. We must know the Genitive singular form, either because we have memorized it or because we have looked it up. This is why a Latin dictionary is not a luxury but a practical necessity. The dictionary gives us what we need: the Nominative singular, the Genitive singular, the gender, and the meaning. When we see dominus, domini, we immediately know the Genitive ends in -i, and therefore the noun is Second Declension and the stem is found by removing -i, leaving domin-. When we see spiritus, spiritus, we immediately know the Genitive ends in -us, and therefore the noun is Fourth Declension and the stem is found by removing -us, leaving spirit-.

If we look at the nouns we have studied so far in the simple dictionary form, we can train ourselves to do two things at once: identify the declension by the Genitive ending, and identify the stem by removing that ending. musa, musae shows the First Declension. Aeneas, Aeneae shows the First Declension. liber, libri shows the Second Declension. dominus, domini shows the Second Declension. templum, templi shows the Second Declension. sermo, sermonis shows the Third Declension. sedes, sedis shows the Third Declension. tempus, temporis shows the Third Declension. civis, civis shows the Third Declension. Now we add spiritus, spiritus as a Fourth Declension noun. We also add cornu, cornus as a Fourth Declension neuter noun. These dictionary entries are not merely references; they are the key that unlocks correct declension.

As before, we continue to keep substantives and adjectives together in our mind, because real Latin constantly requires their agreement. When we say Spiritus Sanctus, “the Holy Spirit,” we are joining a substantive with an adjective. Spiritus is the expressed substantive, and Sanctus is the adjective. Since adjectives have cases just like substantives, both words must agree in gender, number, and case. Thus we say Spiritus Sancti for “of the Holy Spirit,” and Spiritui Sancto for “to the Holy Spirit.” Likewise, if we say fructus suavis, “a sweet fruit,” then in the Dative singular we say fructui suavi, “to a sweet fruit,” and in the plural we say fructus suaves, “sweet fruits,” and fructibus suavibus, “to sweet fruits.” We are not trying to master every detail of adjectives here, but we are keeping the principle of agreement alive so that it feels natural as we proceed.

Now we set the Fourth Declension forms before ourselves in two example nouns, one masculine and one neuter, because these display the whole character of the declension.

We begin with spiritus, which may be treated as masculine, and in practice is often spoken of in both masculine and common terms depending on usage. In the singular we say spiritus as the subject. The Genitive singular is also spiritus, meaning “of a spirit.” The Dative singular is spiritui, meaning “to a spirit.” The Accusative singular is spiritum, meaning “a spirit” as a direct object. The Vocative singular is O spiritus, and the Ablative singular is a spiritu, meaning “from a spirit.” In the plural we say spiritus as the subject, spirituum for “of spirits,” spiritibus for “to spirits,” spiritus as the direct object, O spiritus in address, and a spiritibus for “from spirits.” If we pause and compare this with the Second Declension, we notice a striking mark: the Genitive singular is not -i but -us, and the Dative singular is -ui, which is characteristic of this declension.

We also keep in mind a special note that matters for reading: the blessed name Iesus is declined in an unusual way, with the Accusative in -um, while the other forms are in -u. This is not something to debate; it is something to recognize so we are not surprised when we meet it in sacred texts.

Next we practice a neuter noun of the Fourth Declension, cornu, meaning “a horn.” Here the neuter rule remains fixed as always: the Nominative, Accusative, and Vocative are the same in each number. In the singular we say cornu as the subject, cornus for “of a horn,” cornu for “to a horn,” cornu as the direct object, O cornu in address, and a cornu for “from a horn.” In the plural we say cornua as the subject, cornuum for “of horns,” cornibus for “to horns,” cornua as the direct object, O cornua in address, and a cornibus for “from horns.” We notice here another characteristic mark: neuter Fourth Declension nouns have all their singular case forms in -u, which makes them very easy to recognize once we have seen them a few times.

If we now step back and compare the masculine and neuter patterns, we can hold the Fourth Declension in our memory without strain. Fourth Declension nouns are identified by the Genitive singular ending in -us. Masculine nouns commonly have a Nominative in -us, a Dative singular in -ui, an Accusative singular in -um, and an Ablative singular in -u. Neuter nouns may have a Nominative in -u, and in the singular they keep -u throughout, while in the plural they follow the neuter pattern with -ua in the Nominative, Accusative, and Vocative.

This lesson, then, gives us a declension that is smaller than the Third and easier to hold in the mind, but it also sharpens our discipline in vocabulary study. We must be careful not to trust the Nominative alone, because a noun ending in -us may be Second Declension or Fourth Declension, and only the Genitive singular tells the truth. If we keep that principle firm, the Fourth Declension will take its place quietly alongside the others, and our reading will become steadily easier.

Memory Work

1. How many different terminations are found in the Nominative case of the Fourth Declension?
Two: -us and -u.

2. Decline the Fourth Declension masculine noun hic & haec Spiritus. (State the case name, the Latin form and the English.)
Singulariter: Nominativo, hic & haec Spiritus, a spirit; Genitivo, Spiritus, of a spirit; Dativo, Spiritui, to a spirit; Accusativo, Spiritum, a spirit; Vocativo, O Spiritus, O spirit; Ablativo, a Spiritu, from a spirit. Pluraliter: Nominativo, Spiritus, spirits; Genitivo, Spirituum, of spirits; Dativo, Spiritibus, to spirits; Accusativo, Spiritus, spirits; Vocativo, O spiritus, O spirits; Ablativo, a Spiritibus, from spirits.

In short: Spiritus, Spiritus, Spiritui | Spiritum, O Spiritus, a Spiritu | Spiritus, Spirituum, Spiritibus | Spiritus, O Spiritus, a Spiritibus.

Note: The blessed name Jesus has the Accusative in -um and all other forms in -u.

3. Decline the Fourth Declension neuter noun hoc Cornu (a horn). (State the case name, the Latin form and the English.)
Singulariter: Nominativo, hoc Cornu, a horn; Genitivo, Cornus, of a horn; Dativo, Cornu, to a horn; Accusativo, Cornu, a horn; Vocativo, O Cornu, O horn; Ablativo, a Cornu, from a horn. Pluraliter: Nominativo, Cornua, horns; Genitivo, Cornuum, of horns; Dativo, Cornibus, to horns; Accusativo, Cornua, horns; Vocativo, O Cornua, O horns; Ablativo, a Cornibus, from horns.

In short: Cornu, Cornus, Cornu | Cornu, O Cornu, a Cornu | Cornua, Cornuum, Cornibus | Cornua, O Cornua, a Cornibus.

4. Summary of the Fourth Declension:

Note: Neuter nouns of the Fourth Declension have all singular endings in -u.

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