[1] declension of ūnus, duo and trēs
Image #1: ūnus, -a -um has the same endings as a 1st
/ 2nd declension adjective except in the genitive singular [-īus for
all genders] and dative singular [-ī for all genders]:
Image #2: duo, duae, duo
Image #3: In the declension of trēs, all the endings
are the same for all genders apart from the neuter nominative and accusative:
[2] After trēs none of the numbers decline until you reach the multiples of 100, and the number mīlle (1000):
quattuor |
4 |
quīnque |
5 |
sex |
6 |
septem |
7 |
octō |
8 |
novem |
9 |
decem |
10 |
ūndecim |
11 |
duodecim |
12 |
tredecim |
13 |
quattuordecim |
14 |
quīndecim |
15 |
sēdecim |
16 |
septendecim |
17 |
duodēvīgintī |
18 |
ūndēvīgintī |
19 |
vīgintī |
20 |
trīgintā |
30 |
quadrāgintā |
40 |
quīnquāgintā |
50 |
sexāgintā |
60 |
septuāgintā |
70 |
octōgintā |
80 |
nōnāgintā |
90 |
centum |
100 |
[3] 200 - 900
Take a look at these numbers in Spanish:
200: doscientos
300: trescientos
400: cuatrocientos
There are three points to note:
[i] cientos is plural
[ii] these numbers have masculine and feminine forms i.e.
doscientos and doscientas agreeing with the noun and so
they are acting like adjectives
[iii] Those endings -os and -as are
derived from Latin -ōs, -ās, but the Latin accusative plural, not
the nominative.
The Latin numbers 200 – 900 decline like 1st / 2nd
declension adjectives.
Compare:
1st / 2nd declension adjective in the plural
masculine: magnī
feminine: magnae
neuter: magna
Those endings are used with the numbers:
200: ducentī, -ae, -a; CC
300: trecentī, -ae, -a; CCC
400: quadringentī, -ae, -a; CD [100 less than 500]
500: quīngentī, -ae, -a; D
600: sescentī, -ae, -a; DC
700: septingentī, -ae, -a; DCC
800: octingentī, -ae, -a; DCCC
900: nōngentī, -ae, -a; DCCCC / CM [100 less than 1000]
Image #4: 1st / 2nd declension adjective in the plural
alongside the declension of ducentī showing that the endings
are the same; take a look at the accusatives and there you see where the
Spanish numbers come from.
Additional information on all numbers can be found here:
Additional information on the formation of Roman numerals
can be found here:
https://en.wikipedia.org/wiki/Roman_numerals