1st / 2nd declension adjectives are so-called because they have the same endings as 1st / 2nd declension nouns.
Masculine: magnus (compare: 2nd declension masculine in -us)
Masculine: ruber (compare: 2nd declension masculine in -er)
Feminine: magna (compare: 1st declension nouns in -a)
Feminine: rubra (note the loss of /e/ when endings are added)*
Neuter: magnum (compare: 2nd declension nouns in -um)
Neuter: rubrum (note the loss of /e/ when endings are added)*
*Not all adjectives in -er lose the /e/ when endings are added:
līber, lībera, līberum (free)
miser, misera, miserum (wretched; unhappy)
1st / 2nd declension adjectives are listed in shorthand: magnus, -a, -um. However, they are also called ‘2-1-2’ adjectives. As soon as you see that term, you know you are dealing with this type of adjective in -us, -a, -um.
In the same way that nouns can be grouped according to topic, so too can adjectives be categorised according to the type of description that they give. For example:
[i] size
magnus: big; great
parvus: small
[ii] shape
rotundus: round; circular
[iii] length
longus: long; far
[iv] height; depth; width
altus: high; tall; deep
angustus: narrow
lātus: wide
[v] character
amīcus: friendly
benignus: kind
ignāvus: lazy; cowardly
piger: lazy; backward
[vi] mood
irātus: angry
laetus: happy
[vii] colour
albus: white
caeruleus: blue
ruber: red
cānus: grey-haired
[viii] appearance
pulcher: beautiful
[ix] age
antīquus: old; ancient
[x] material
ligneus: wooden
aureus: golden
The 1st / 2nd declension adjectives have exactly the same endings as the nouns. Posted is a table summarising those endings.
Some points to remember:
[1] first declension nouns which are male e.g.agricola (farmer), nauta (sailor) have masculine adjectives:
agricola ignāvus: a lazy farmer
nautae irātī: angry sailors
This would come as no surprise to a Russian speaker; the noun dedushka (grandfather) is grammatically feminine in Russian, but the adjective that agrees with it is masculine!
There are some 2nd declension nouns - not many - which are feminine. These, too, will have a feminine adjective agreement:
quercus (fem.) oak
> quercus rubra: red oak
[2] These 2-1-2 adjectives always have these endings. They are not simply imitating the endings of the nouns which they describe; they just happen to share them. The examples below show how the ending of the noun and the ending of the adjective function separately. To illustrate this we will use:
[i] two adjectives which mean more or less the same:
ignāvus, -a, -um (1st/2nd declension adj. in -us): lazy
piger, pigra, pigrum (1st/2nd declension adj. in -er): lazy
[ii] two nouns which mean almost the same:
puerulus (1st/2nd declension noun in -us): little boy
puer (1st/2nd declension noun in -er): boy
Now look at how the adjectives and nouns work together.
[i] Nominative singular: puer ignāvus; the adjective ending remains -us even if the noun has a different ending.
> Vocative singular: puer ignāve!; nouns ending in -er have no separate vocative case, but adjectives ending in -us do.
[ii] Nominative singular: puerulus piger; the adjective ending remains -er even if the noun has a different ending.
> Vocative singular: puerule piger!; nouns ending in -us have a separate vocative case, but adjectives ending in -er do not.
Bearing this in mind becomes increasingly important since, when you begin to study nouns of other declensions, you will see that they have a range of endings, but if first / second declension adjectives are used with them, the adjectives will retain the same endings as shown here.
Masculine: rēx magnus (a great king)
Feminine: urbs antīqua (an ancient city)
Neuter: iter longum (a long journey)
Similarly, there is a second group of adjectives (to be discussed in a later post) which decline differently from 1st / 2nd declension adjectives, and they too will retain their endings even if they are being used to describe a 1st / 2nd declension noun. Therefore, draw this conclusion: the noun will determine the case, gender and number of the adjective which modifies it, but it will not dictate what the ending of the adjective will be because that is determined by the type of adjective. The declensions of nouns and adjectives must be studied separately.