Don’t worry—it’s extinct! However, it’s insightful to see that ambō the long ō ending that is characteristic of Greek duals and dual-related adverbs: ἄμφω, δύω, κτλ.
The Essential AG: p.59, ftn.
Ambō is sometimes decline to match its respective noun, like a fully-functional adjective, but otherwise remains fixed as ambō
- Ambās mānūs lavit: he washed both hands.
- Consulēs alter ambōve prōgredientur: either one or both of the consuls will march on
- ūna salus ambobus erit: both were healthy (this is Virgil—a very Greek declension!)
Effectively, there is no rule. Both A&G and L&S present freeform variation between the two options.
There are four central aspects to the Latin numeral:
- The cardinal: ūnus, duo, trēs, quattuor
- The ordinal: prīmus, secundus, tertius, quārtus
- The distributive: singulī, bīnī, ternī, quaternī
- The adverb: semel, bis, ter, quater
From 1-10, only cardinals 1, 2, and 3 decline.
A few things to consider:
- ūnus will often mean ‘only’ (cf. sōlus) and occasionally ‘the same’ (cf. idem)
- where ūnus means ‘only,’ it may initiate a subjunctive clause of characteristic (the only man who may: ūnus cuī liceat.)
- the compound ūnus quisque = every single one
- the compound ūnus + superlative = the one most (the one most learned man, ūnus doctissiumus)
- duo may also have the plural genitive duum
- the word ambō (both, which retains the long ō of the lost Latin dual) declines like duo
- the compound ūnus + superlative = the one most (ūnus doctissiumus, the one most learned man)
Here’s a chart I found showing the descendents of the Latin cardinals:
(courtesy N.S. Gill; http://tiny.cc/eoiqmw)(For those of you who are curious, there are between 30 and 40 standing Romance languages, but we’ll get to numbers above 10 next post…)
The Essential AG: 133-4