A&G define the vocative as “the case of Direct Address.” (35f)
Generally speaking, the vocative and the nominative are the same.
However, in certain nouns of the second declension (those with nominative -us or -ius) have two exceptional variations. All nouns in -us feature an -e in the vocative (mūrus…mūre). Those ending in -ius (Vergilius, fīlius, genius, etc.) take a vocative -ī (Vergilī, filī, genī).
[Highly attentive readers should note that this vocative does not shift its accent, rendering Vergílī, and not *Vérgilī, as one might expect.]
That’s how it stands for nouns. There’s a slight variation in policy for adjectives, though luckily the same general rule (same as the nominative) holds true for all but the second declension (bonus…bone). However, the one catch is that adjectives ending in -ius change to -ie and not -ī. Therefore, when calling to a Spartan son, we might say O fīlī Lacedaemonie! (not *Lacedaemonī).
If anyone has a better understanding of vocative plurals, which I assume are all identical to their nominative forms, feel free to say more in the comments below. A&G are totally silent on this issue, which I assume signals that listing the vocatives would be redundant (with respect to the nominatives).
Some of the more common impersonal expressions in Latin are those that describe the weather.
grandinat, it’s hailing
pluit, it’s raining
ningit, it’s snowing
fulgurat, there’s a lightning bolt! (A&G have ‘it lightens’)
tonat, it thunders
rōrat, there’s dew on the grass
Note that these verbs can take subjects (Iupitter tonat) but they don’t have to. A&G are incomplete here, so let’s try to extrapolate on other ways the Romans might have describe the weather. I’m working with the assumption that these impersonal expressions are much like those of modern Romance languages (hace calor, fa caldo, it’s warm). The Latin expressions ought to cover the same range, right?
calidum est, it’s warm
frigidum est, it’s chilly
hūmidum est, it’s humid
nubiliōsum est, it’s cloudy
partim nubiliōsum est, it’s partly cloudy
ventōsum est, it’s windy
lūcet, it’s sunny
partim lūcet, it’s partly sunny
The impersonal list in A&G technically covers ‘verbs expressing operations of nature and the time of day,’ so here are two more entries in the list: