Latin Reference — Vol. III: Adjective Agreement
The three-way rule
An adjective agrees with its noun in case, number and gender — and nothing else. Crucially, it does not have to be in the same declension, and the agreeing forms do not have to look alike. Agreement is grammatical, not cosmetic.
First/second declension adjectives — magnus, -a, -um, great
These borrow their endings wholesale: masculine from the second declension, feminine from the first, neuter from second-declension neuters.
| Case | M. sing. | F. sing. | N. sing. | M. pl. | F. pl. | N. pl. |
|---|---|---|---|---|---|---|
| Nom | magnus | magna | magnum | magnī | magnae | magna |
| Gen | magnī | magnae | magnī | magnōrum | magnārum | magnōrum |
| Dat | magnō | magnae | magnō | magnīs | magnīs | magnīs |
| Acc | magnum | magnam | magnum | magnōs | magnās | magna |
| Abl | magnō | magnā | magnō | magnīs | magnīs | magnīs |
Third declension adjectives — fortis, forte, brave/strong
Two-termination: one form serves masculine and feminine, a second serves neuter. Note the ablative singular in -ī (not -e as in third-declension nouns).
| Case | M./F. sing. | N. sing. | M./F. pl. | N. pl. |
|---|---|---|---|---|
| Nom | fortis | forte | fortēs | fortia |
| Gen | fortis | fortis | fortium | fortium |
| Dat | fortī | fortī | fortibus | fortibus |
| Acc | fortem | forte | fortēs | fortia |
| Abl | fortī | fortī | fortibus | fortibus |
Cross-declension agreement
māter is third declension but feminine, so it takes the feminine forms of magnus — first-declension-looking endings on a third-declension noun's adjective:
| Case | Phrase | Translation |
|---|---|---|
| Nom | māter magna | the great mother |
| Gen | mātris magnae | of the great mother |
| Dat | mātrī magnae | to the great mother |
| Acc | mātrem magnam | the great mother (obj.) |
| Abl | mātre magnā | with/by the great mother |
The endings don't match visually (mātrem … magnam) — but case, number and gender all agree. This is the single biggest conceptual hurdle; once it stops looking wrong, agreement is understood.
Worked pair — mīles fortis, the brave soldier
Both happen to be third declension here, yet the forms still aren't identical twins:
| Case | Phrase |
|---|---|
| Nom | mīles fortis |
| Gen | mīlitis fortis |
| Dat | mīlitī fortī |
| Acc | mīlitem fortem |
| Abl | mīlite fortī |
Note the ablative: noun in -e, adjective in -ī — agreement without lookalike endings, again.
Why agreement frees the word order
Because the endings declare which adjective belongs to which noun, Latin can scramble word order for emphasis without ambiguity:
Fortis rēgem mīles videt.
Parse by endings, not position: fortis (nom.) pairs with mīles (nom.) — the brave soldier; rēgem is accusative — the object. So: the brave soldier sees the king, even though fortis sits next to rēgem on the page. The case endings, not the neighbours, tell you who goes with whom.
This is the payoff of the whole case system: word order in Latin carries emphasis, while the endings carry grammar. English does it the other way round.