Ea dicit plerosque vicinos ob imbrem et pruinae frigus domi mansisse.

Because after dicit in this sentence, Latin is using an indirect statement construction.

Instead of saying:

• She says that most neighbors stayed at home

Latin often says:

• She says most neighbors to have stayed at home

That sounds odd in English, but in Latin it is normal. In this construction:

• the subject of the reported statement goes into the accusative
• the verb of the reported statement goes into the infinitive

So:

• plerosque vicinos = most neighbors (accusative subject of the indirect statement)
• mansisse = to have remained / to have stayed

This is why you do not get nominative plerique vicini here.

For the same reason: dicit introduces an indirect statement.

Latin commonly uses:

• verb of saying/thinking/perceiving
  • accusative subject
    • infinitive verb

So in this sentence:

• Ea dicit = She says
• plerosque vicinos ... mansisse = that most neighbors stayed / have stayed ...

The infinitive mansisse is the perfect infinitive of maneo, manere.

So literally the structure is:

• She says most neighbors to have stayed at home because of the rain and the cold of the frost.

Natural English then becomes:

• She says that most of the neighbors stayed at home because of the rain and the cold frost.

Mansisse is the perfect active infinitive of maneo.

Its parts are:

• maneo = I remain, stay
• manere = to remain, to stay
• mansi = I remained, I stayed
• mansisse = to have remained, to have stayed

In indirect statement, the perfect infinitive usually shows action prior to the main verb. So:

• Ea dicit ... mansisse literally means She says ... to have stayed

In smoother English, that often becomes:

• She says that ... stayed
• or sometimes has stayed, depending on context

So the perfect infinitive tells you the staying happened before or by the time of the saying.

Here Ea most naturally means she.

It is the feminine singular form of the pronoun is, ea, id. Depending on context, ea can mean different things, such as:

• she
• that woman
• those things (if neuter plural context is not involved here)
• these things / those things, depending on context

But in this sentence, because dicit is singular and the rest of the sentence suggests a person speaking, Ea is best understood as:

• She says ...

Plerosque means most or very many.

It comes from plerique, pleraeque, pleraque, a word that is often translated as:

• most
• the majority of

Here it agrees with vicinos:

• plerosque vicinos = most neighbors or most of the neighbors

Both words are:

• masculine
• plural
• accusative

because they are functioning together as the accusative subject of the indirect statement.

Ob is a preposition that takes the accusative case and usually means:

• because of
• on account of

So:

• ob imbrem = because of the rain

The noun here is imber, imbris (rainstorm, rain), and its accusative singular is:

• imbrem

That is why you see imbrem, not imber.

Because pruinae is not directly governed by ob here. Instead, it depends on frigus.

The phrase is:

• pruinae frigus = the cold of frost / the cold caused by frost

So the structure is:

• ob imbrem = because of the rain
• et pruinae frigus = and the cold of frost

Here pruinae is genitive singular, and frigus is the main noun of that phrase.

So ob directly governs only imbrem. The second idea is added by coordination:

• because of the rain and the frost-cold

A learner might expect something like ob imbrem et pruinam, but that would mean because of the rain and the frost. This sentence is slightly more specific: it points to the cold of frost.

Pruinae is genitive singular.

It depends on frigus:

• frigus = cold
• pruinae frigus = the cold of frost

This is a very common Latin pattern: one noun in the genitive limits or describes another noun.

Other English-style ways to understand it are:

• frost’s cold
• the cold of the frost
• frost-cold

So pruinae is not a separate object of ob. It is attached to frigus.

Domi means at home.

It is a special locative form of domus. A few Latin nouns preserve this older case for place where, and domi is one of the most common examples.

So:

• domi = at home

This is different from:

• in domo = in the house

Those are similar, but not always identical in feel:

• domi usually means at home in the ordinary, idiomatic sense
• in domo is more literally in the house/building

So domi mansisse is the normal Latin way to say:

• to have stayed at home

Latin word order is much more flexible than English word order. The ending -isse on mansisse already tells you it is the verb of the indirect statement, so Latin does not need to place it in one fixed position.

Putting mansisse at the end is very natural Latin style. It lets the sentence build toward the main action:

• Ea dicit
• plerosque vicinos
• ob imbrem et pruinae frigus
• domi mansisse

So the sentence ends with the key idea:

• to have stayed at home

This is normal and often elegant in Latin.

Vicinos comes from vicinus, which means:

• neighboring
• nearby
• as a noun, neighbor

So plerosque vicinos is most naturally:

• most of the neighbors

Depending on context, it could also suggest:

• most nearby people
• most of the people living nearby

But neighbors is the standard translation.

Yes: the most important thing is to recognize the indirect statement quickly.

A useful way to break the sentence up is:

• Ea dicit = main clause
• plerosque vicinos ... mansisse = indirect statement
• ob imbrem et pruinae frigus = reason
• domi = place where

So the grammar works like this:

• Ea = subject of dicit
• dicit = main verb
• plerosque vicinos = accusative subject of the infinitive
• mansisse = perfect infinitive
• ob imbrem et pruinae frigus = explains why they stayed
• domi = where they stayed

If a learner can spot that pattern, the whole sentence becomes much easier to read.