mfranks wrote:Why is 'inter' and 'interest' used in the first clause of the sentence? Both words mean "between".
It's very common in Latin to use both a compound verb and a preposition (particularly the same preposition already in the compound) where in English we would expect either a compound verb or a verb followed by a preposition, but not both. Part of this is because English likes to form complex verbs by adding a preposition after the basic verb, instead of using a prefix the way Latin does, so that prepositional phrases and verbs mush together.
Thinking of analogous forms, you would have in Latin "expellere virum ex urbe" -- "to expel the man from the city", or "to drive (out) the man out of the city". Even though the verb is compounded, you still have to use a preposition before "city" because it isn't the object of the verb. You could say "pellere virum ex urbe", I suppose, but Latin likes the extra emphasis. It seems unnatural in English, but if you see enough of it in Latin, I think you'll find that the redundancy starts to seem elegant.
Tum naves et nautae in mare merguntur.
Then the boat and the sailors plunged into the sea.
How is this passive?
Isn't the following equivalent?
Tum naves et nautae in mare mergent.
I think this is a case where the English verb (to plunge, immerse) can be used a bit differently than the Latin. I suspect the Latin verb is transitive -- you have to use it with a direct object. So to be active you would expect: naves nautas in mare mergent
(the ships plunged the sailors into the sea). In your sentence, the sailors aren't causing the plunging, some unknown force or agent is. So they are "being plunged", passive.
I've noticed in a lot of the Neo Latin books of Children's Classics such as Ferdinandus Taurus or Virent Ova! Viret Perna!! for example, they use alot of the passive forms of the verbs without ab/a.
Not having examples, I don't know exactly what you're seeing, but there are several points worth mentioning:
1) Be careful not to confuse the ablative of agent (which is a person and requires a/ab) with the ablative of instrument or means (which is a thing by which something is brought about and does not use a preposition)
To use the previous example:
Naves et nautae a Iove [by Jupiter] in mare merguntur (abl. of agent)
Naves et nautae undiis [by means of waves] in mare merguntur (abl of means)
2) Passive periphrastics use a different construction (dative of a person) instead of the ablative of agent.
3) There are certain verbs which are deponent -- they have passive forms but active meanings.