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Lesson 19. The Fourth Declension of Nouns

To complete this lesson, complete the following tasks:

  1. Study the Lesson for mastery.
  2. Complete all assignment Memory Work.
  3. Complete all lesson Assessments.


In lesson 03, we were introduced to the “accidents” of the parts of speech. In that lesson, I said: “In this lesson, we will learn a little about each of these accidents, and then we will study them in detail throughout the rest of the course.” In this lesson, we begin our detailed study of the declensions of Latin nouns.

In lesson 03, we learned that he Declensions of nouns are five.

  • The First declension, whose genitive singular is ended with the diphthong -AE, as musa, musae.
  • The Second declension, whose genitive singular is ended with the letter -I , as dominus, domini.
  • The Third declension, whose genitive singular is ended with (the) syllable -IS , as sermo, sermonis.
  • The Fourth declension, whose genitive singular is ended with the syllable -US , as sensus, sensus.
  • The Fifth declension, whose genitive singular is ended with the letters E and I, as dies, diei.

In this lesson, we will study the forms of the Fourth Declension nouns.

Example Nouns of the Fourth Declension

Latin nouns of the fourth declension have their nominative case ending in -US or -U, and are declined as follows. We will study two example nouns of the fourth declension:

  • hic sensus, sensus (sense)
  • hoc genu, genu (knee)


The first example noun of the fourth declension is hic sensus. The pronoun hic marks the gender as masculine, and the noun is declined as follows:


Nominative: hic sensus, (the) sense
Genitive: sens-us of (the) sense
Dative: sens-ui, to (the) sense
Accusative: sens-um, (the) sense
Vocative: O sensus, O (the) sense
Ablative: a sens-u, from (the) sense


Nominative: hi sens-us, (the) senses
Genitive: sens-uum of (the) senses
Dative: sens-ibus, to (the) senses
Accusative: sens-us, (the) senses
Vocative: O sens-us, O senses
Ablative: a sens-ibus, from (the) senses

Important: We see that the ending of the genitive singular form is -US, which marks this declension. Realize that you will need to distinguish these in reading from second declension nouns ending in -us as dominus.


The second example noun of the third declension is the noun hoc genu. The pronoun hoc marks this as a neuter noun, and it is declined as follows:


Nominative: hoc genu, (the) knee
Genitive: gen-u of (the) knee
Dative: gen-u, to (the) knee
Accusative: gen-u, (the) knee
Vocative: O gen-u, O knee
Ablative: a gen-u, from (the) knee


Nominative: haec gen-ua, (the) knees
Genitive: gen-uum of (the) knees
Dative: gen-ibus, to (the) knees
Accusative: gen-ua, (the) knees
Vocative: O gen-ua, O knees
Ablative: a gen-ibus, from (the) knees

Important: Note that genitive singular of neuter nouns of the fourth declension is -U and not -US. Nevertheless, only fourth declension has a genitive ending based on the vowel U, so they are easy to identify.

Exceptions in -UBUS

In the dative and ablative plural, the nouns arcus, artus, lacus, partus, specus and tribus end in -UBUS rather than -IBUS, as artubus, tribubus, etc. The noun portus uses both -IBUS and -UBUS. Therefore, know that any nouns found ending in -UBUS belong to the fourth declension.

Substantives and Adjectives

After learning the forms of the example nouns of the fourth declension, we must give attention to the declension of substantives with adjectives. Adjective nouns have cases just like substantives, so when they are used together, they must always be written with the same gender, numbichaeler and case. Here are some examples to be declined:

  • hic sensus hebes
  • his gradus tardus
  • hic sensus tenuis
  • his census tenuis
  • haec anus delira
  • hoc genu tumens
  • hoc veru subtile
  • hoc cornu inflexum
  • hoc tonitru horrendum

First, let us consider hic sensus, with an adjective of the third declension:


Nominative: hic sensus hebes, dull sense
Genitive: sens-us hebet-is of dull sense
Dative: sens-ui hebet-i, to dull sense
Accusative: sens-um hebet-em, dull sense
Vocative: O sensus hebes, O dull sense
Ablative: a sens-u hebet-i, from dull sense


Nominative: hi sens-us hebet-es, dull senses
Genitive: sens-uum hebet-ium of dull senses
Dative: sens-ibus hebet-ibus, to dull senses
Accusative: sens-us hebet-es, dull senses
Vocative: O sens-us hebet-es, O dull senses
Ablative: a sens-ibus hebet-ibus, from dull senses

Now, let us consider the neuter fourth declension noun hoc tonitru with an adjective of the second declension. Notice that, while the endings differ, the gender, number and case they signify remain the same:


Nominative: hoc tonitru horrend-um, (the) knee
Genitive: tonitr-u horrend-i of (the) knee
Dative: tonitr-u horrend-o, to (the) knee
Accusative: tonitr-u horrend-um, (the) knee
Vocative: O tonitr-u horrend-um, O knee
Ablative: a tonitr-u horrend-o, from (the) knee


Nominative: haec tonitr-ua horrend-a, (the) knees
Genitive: tonitr-uum horrend-orum of (the) knees
Dative: tonitr-ibus horrend-is, to (the) knees
Accusative: tonitr-ua horrend-a, (the) knees
Vocative: O tonitr-ua horrend-a, O knees
Ablative: a tonitr-ibus horrend-ibus, from (the) knees

Remember, substantives and adjectives from different declensions may be joined, but they must agree in gender, number and case. They may not look the same, but their characteristics must be the same.

Memory Work

To learn the declensions, you must rely on memorization. Recite the questions and answers below until they are thoroughly memorized. Rely on the lesson assessments to test your progress.

  1. How many different terminations are found in the Nominative case of the Fourth Declension?
    Two: -us, -u.
  2. Decline the Fourth Declension masculine noun hic Sensus (sense). (State the case name, the Latin form and the English.)
    Singular: Nominative, hic Sensus, sense; Genitive, Sensus, of sense; Dative; Sensui, to sense, Accusative, Sensum, sense; Vocative, O Sensus, O sense; Ablative, a Sensu, from sense. Plural: Nominative, Sensus, senses; Genitive, Sensuum, of senses; Dative, Sensibus, to senses; Accusative, Sensus, senses; Vocative, O Sensus, O senses; Ablative, a Sensibus, from senses.

In short: Sensus, Sensus, Sensui, Sensum, O Sensus, a Sensu; Sensus, Sensuum, Sensibus, Sensus, O Sensus, a Sensibus.

  1. Decline the Fourth Declension neuter noun hoc Genu (knee). (State the case name, the Latin form and the English.)
    Singular: Nominative, hoc Genu, (a) knee; Genitive, Genu, of (a) knee; Dative, Genu, to (a) knee; Accusative, Genu, (a) knee; Vocative, O knee, O time; Ablative, a Genu, from (a) knee. Plural: Nominative, Genua, knees; Genitive, Genuum, of knees; Dative, Genibus, to knees; Accusative, Genua, knees; Vocative, O Genua, O knees; Ablative, a Genibus, from knees.

In short: Genu, Genu, Genu, Genu, O Genu, a Genu; Genua, Genuum, Genibus, Genua, O Genua, a Genibus.

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