Chapter 16 Must-knows

Big rule #1:
nouns and adjectives agree in case/ number/ gender
The same ending does not always mean the same case
Which adjectives are 1-, 2-, or 3-endings

3rd declension adjectives

So far we know three declensions of nouns and one category of adjective:

Nouns
1st, gen. sing. ae
2nd, gen. sing. ī
3rd, gen. sing. is

Adjective
1st/2nd, gen. sing. f. ae; gen. sing. m. ī; gen. sing. n. ī.

3rd declension adjectives are the only other (main) group of adjectives. Once you know 1st/2nd and 3rds, you know almost all the adjectives you will run across in Latin. The tricky part is that the extra group makes it a little more confusing to match noun-ending and adjective-ending.

Luckily, there are no new endings to learn, though. 3rd declension adjectives decline exactly the same as 3rd declension i-stems (remember them?).

Remember, 3rd declension i-stem means:
ablative sing. -ī
genitive pl. -ium
neuter nom. and acc. pl. -ia

The one other thing about this group of adjectives is the funny nom. sing. We know the nominative in 3rd declension nouns can look anywhichway. This case is peculiar in the adjectives too: in the nom. sing. of the 3rd declension, adjectives can have

three forms (one for each of the three genders)
two forms (the first form for masc. and fem., and a second form for neuters)
one form (for all three genders)

These are called 3-ending, 2-ending, and 1-ending adjectives (for obvious reasons). Other than that, there is nothing new here: this group acts just like our other adjectives, follows the same rules, works the same in sentences, etc.

See p. 105 for the complete declensions of these new adjectives, and also look on p. 106 at the pairs at the top of the page (omnis amīcus through omnium marium).