Lucia autem censet se illo die et de hereditate et de alveo fluminis multa didicisse.

Because after verbs like censet (thinks, judges, believes), Latin very often uses an indirect statement construction.

That construction is:

• accusative subject
  • infinitive

So here:

• censet = she thinks
• se = herself / that she
• didicisse = to have learned

Together, censet se ... didicisse means she thinks that she learned / has learned ...

Latin usually does this where English uses that.

Se is the reflexive pronoun, meaning it refers back to the subject of the main verb.

Here the main subject is Lucia, and the sentence says that Lucia thinks something about herself. So Latin uses se:

• Lucia censet se didicisse = Lucia thinks that she learned

If Latin used eam, that would normally mean her as someone else, not Lucia herself.

So:

• se = Lucia herself
• eam = some other woman

Didicisse is the perfect active infinitive of disco, discere, didici (to learn).

It means to have learned.

In an indirect statement, the tense of the infinitive shows time relative to the main verb:

• present infinitive = action happening at the same time
• perfect infinitive = action earlier than the main verb
• future infinitive = action later than the main verb

So in this sentence:

• censet = she thinks (present)
• didicisse = to have learned (earlier)

That is why the sense is something like:

• Lucia thinks that she learned
• or more literally, Lucia thinks herself to have learned

Because Latin infinitive tenses in indirect statement show relative time, not always the exact English tense you must use.

Didicisse tells you that the learning happened before the thinking. English can express that in different ways depending on context:

• she thinks that she learned
• she thinks that she has learned
• sometimes even she thinks that she had learned

In this sentence, illo die (on that day) makes it clear the learning happened on a specific earlier day, so she thinks that she learned ... on that day is very natural.

Illo die is ablative singular, and it expresses time when.

• ille, illa, illud = that
• dies, diei = day
• illo die = on that day

This is a very common Latin use of the ablative: no preposition is needed for a point in time.

Examples of the same idea:

• eo anno = in that year
• tertia hora = at the third hour
• illo die = on that day

This is a standard Latin pattern:

• et ... et ... = both ... and ...

Latin often repeats the preposition with each item, especially when the phrasing is balanced and clear:

• et de hereditate et de alveo fluminis
• both about the inheritance and about the riverbed

Latin could sometimes omit the second de in other contexts, but repeating it is very normal and often stylistically neat.

De takes the ablative case.

So here:

• de hereditate = about the inheritance
• de alveo = about the riverbed / channel

This is one of the first things learners memorize with prepositions: certain prepositions always govern certain cases, and de governs the ablative.

It breaks down like this:

• de = about / concerning
• alveo = ablative singular of alveus
• fluminis = genitive singular of flumen

So:

• alveo = riverbed / channel in the ablative because of de
• fluminis = of the river

Together:

• de alveo fluminis = about the riverbed of the river

In smoother English, that is often just about the riverbed.

Because it depends on alveo and tells you whose/what kind of bed it is:

• alveus fluminis = the bed/channel of the river

The genitive often works like English of.

So:

• fluminis = of the river

Multa is the neuter plural accusative of multus, -a, -um, and here it means:

• many things
• or more naturally, much

It is the direct object of didicisse:

• multa didicisse = to have learned many things / much

Latin often uses neuter plural adjectives by themselves as pronouns or substantives:

• multa = many things
• pauca = a few things
• omnia = everything / all things

So the sentence says Lucia thinks she learned a lot about those subjects.

Because Latin commonly expresses many things with a neuter plural adjective used substantively.

So multa does not mean many with a noun omitted in an unusual way; this is a very normal Latin idiom:

• multa dicere = to say many things / a lot
• multa scire = to know much / many things
• multa didicisse = to have learned much / many things

English often translates it simply as much or a lot.

Autem usually means something like:

• however
• but
• moreover
• on the other hand

It is a postpositive word, which means it normally comes after the first word or phrase, not first in the sentence.

So:

• Lucia autem censet ...

means something like:

• Lucia, however, thinks ...
• But Lucia thinks ...

A learner should notice that Latin word order here is normal for autem: it comes second, not first.

The word order is flexible, but nothing here is especially strange.

A rough layout is:

• Lucia autem censet
• se
• illo die
• et de hereditate et de alveo fluminis
• multa didicisse

Latin often puts the infinitive at or near the end in indirect statement, and it often places time expressions and prepositional phrases before it.

So the order is quite natural:

• subject + autem
• main verb
• accusative subject of infinitive
• time phrase
• things learned about
• object + infinitive

Grammatically, a learner might wonder that, but in context it naturally goes with didicisse.

So the sense is:

• she thinks that on that day she learned much ...

rather than:

• on that day she thinks that she learned much ...

Latin word order can leave some temporary ambiguity, but meaning and common sense usually resolve it. Here illo die is most naturally the time of the learning.

A learner would probably want to identify these forms:

• censet ← censeo, censere, censui, censum = think, judge
• didicisse ← disco, discere, didici = learn
• hereditate ← hereditas, hereditatis = inheritance
• alveo ← alveus, alvei = channel, trough, riverbed
• fluminis ← flumen, fluminis = river
• illo ← from ille, illa, illud
• die ← dies, diei

Seeing the dictionary forms helps explain the cases and syntax.

Very literally, it is something like:

• Lucia, however, thinks herself on that day to have learned many things both about the inheritance and about the riverbed of the river.

That is awkward English, but it shows the Latin structure clearly:

• censet = thinks
• se = herself
• didicisse = to have learned
• multa = many things
• et ... et ... = both ... and ...

A more natural English translation would be:

• Lucia, however, thinks that on that day she learned a great deal both about the inheritance and about the riverbed.