1. Essentials at a Glance

Third declension adjectives in Latin are adjectives that take third-declension endings. They can have one, two, or three nominative singular forms depending on gender distinctions (e.g., ācer, ācris, ācre). They frequently follow i-stem rules—showing ablative singular -ī, genitive plural -ium, and neuter plural -ia. These forms matter because they appear everywhere in classical texts, and understanding them is crucial for reading, writing, and accurately translating Latin.

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2. Definition & Importance

Definition:
A third declension adjective is any adjective in Latin whose stem behaves like a third-declension (often i-stem) noun. They may have one, two, or three different nominative singular endings for masculine, feminine, and neuter.

Importance:

• They are central to Latin syntax and morphology, as they attach third-declension endings to agree with nouns of all genders and numbers.
• Latin 3rd declension adjective examples (e.g. fortis, forte = “brave”) illustrate how these adjectives modify nouns in both prose and poetry.
• Common errors in 3rd declension adjectives often involve incorrect ablative or plural endings; knowing these forms is pivotal for advanced reading comprehension.

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3. Forms & Morphology

Third declension adjectives fall into three main categories:

Three-Termination Adjectives

• Example: ācer, ācris, ācre (“sharp”)
• They have distinct masculine, feminine, and neuter nominatives.
• i-stem endings appear in abl. sg. (-ī), gen. pl. (-ium), neut. pl. (-ia), and often acc. pl. (-īs) for m./f.

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Two-Termination Adjectives

• Example: fortis, forte (“strong, brave”)
• One form for masculine/feminine (fortis) and another for neuter (forte).
• They predominantly use i-stem endings across the board.

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One-Termination Adjectives

• Example: atrōx, atrōcis (“fierce”); sapiēns, sapientis (“wise”).
• A single nominative form for all genders, but neuters still follow the usual neuter rule in accusative/nominative.
• Most have i-stem endings, though a few behave like consonant stems (e.g., vetus shows vetere, veterum).

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4. Usage & Examples

When Used and How:

• Third declension adjectives agree with nouns in gender, number, and case.
• They can be used attributively (e.g., vir fortis) or predicatively (e.g., vir est fortis).
• They frequently function as substantives, standing alone to mean “the [adjective] ones” (e.g., fortēs = “brave [people]”, omnia = “all things”).

Classical Latin Examples

1. Cicero, De Officiis I.20

   • “Fortis… esse certē nullō modō potest.”
   • Translation: “A man cannot in any way be brave if…”
   • Note: fortis (m./f. nominative) is used substantively to describe a person’s virtue.

2. Caesar, De Bello Gallico 1.1

   • “Gallia est omnis dīvīsa in partēs trēs…”
   • Translation: “Gaul as a whole is divided into three parts…”
   • Note: omnis (m./f. nom. sg.) modifies Gallia, emphasizing the entire territory.

3. Vergil, Aeneid 10.284

   • “Audentīs Fortūna iuvat.”
   • Translation: “Fortune favors the bold.”
   • Note: audentīs is accusative plural (substantive use of the participle/adjective “the daring [ones]”).

4. Horace, Odes 2.3

   • “Dīvesne prīscō nātus ab Īnachō / nīl interest, an pauper…”
   • Translation: “It matters not whether you were born rich from ancient Inachus or a poor man…”
   • Note: dīves and pauper are one-termination adjectives used substantively in contrast.

5. Tacitus, Historiae I.49

   • “omnium cōnsēnsū capax imperiī, nisi imperāsset.”
   • Translation: “By universal consent, [he seemed] capable of rule—if only he had never ruled.”
   • Note: capax imperiī (capable of power) illustrates how some adjectives take a genitive (imperiī).

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5. Common Pitfalls

• Mixing -ī and -e in the Ablative Singular
  • Regular prose uses -ī for i-stem adjectives (e.g., fortī mente). -ē appears in poetry or archaic texts.
• Forgetting Neuter Plural Endings
  • -ia (nom./acc. pl.) and -ium (gen. pl.) are often overlooked.
• Confusing One-Termination Forms
  • Ensure you correctly identify gender from context (atrōx can be m., f., or n.).
• Defective or Irregular Words
  • pauper, inops, and others may lack certain forms (especially neuter plurals).

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6. Additional Notes & Nuances

• Semantic Restrictions:
  Some third declension adjectives (e.g., pauper, inops) seldom use neuter plural because they inherently describe people or personal states.
• Historical Variants:
  Early Latin sometimes used the feminine form for masculine nouns (e.g., coetus alacris in Ennius). Classical usage later standardized distinct masculine forms.
• Participle-Adjective Overlap:
  Present participles (e.g., audiēns) function as 3rd declension adjectives but can adopt slightly different ablative forms (-ī vs. -ē) depending on context (purely adjectival vs. verbal/participial).
• Comparatives as Consonant Stems:
  All comparatives in -ior/-ius (e.g., fortior, fortius) follow third declension but often use consonant-stem endings in the ablative (-e) and genitive plural (-um).

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7. Key Takeaways

• Recognize i-Stem Endings:
  Look for ablative in -ī, neuter plural in -ia, and genitive plural -ium.
• Check the Nominative Singular Form(s):
  Identify whether an adjective is one-, two-, or three-termination for proper agreement.
• Watch for Defective Forms:
  Some adjectives lack certain genders or numbers for semantic reasons (e.g., pauper rarely has a neuter plural).
• Use Context for Gender Clues:
  One-termination adjectives (like ingēns) rely on the noun’s own gender endings for clarity.
• Mind the Syntax:
  Third declension adjectives can be used substantively or govern a genitive (capax imperiī).