Lucia dicit comparativum unam rem cum alia conferre, superlativum autem unam rem omnibus praeferre.

Because after dicit Latin is using indirect statement, also called the accusative-and-infinitive construction.

So instead of saying:

• Lucia says the comparative compares ...
• Lucia says the superlative prefers ...

Latin says, more literally:

• Lucia says the comparative to compare ...
• Lucia says the superlative to prefer ...

That is why the verbs inside the reported statement are infinitives: conferre and praeferre.

The subjects are comparativum and superlativum.

In English, the subject of an infinitive is often not expressed clearly, but in Latin it can be. In indirect statement, that subject appears in the accusative.

So the two parts are:

• comparativum ... conferre = that the comparative compares
• superlativum ... praeferre = that the superlative prefers

They are neuter singular forms meaning the comparative and the superlative.

A useful detail: in neuter nouns and adjectives, the nominative singular and accusative singular are the same form. So here, because they are in indirect statement, they function as accusative subjects of the infinitives, but the form still looks exactly like a nominative would.

So comparativum and superlativum are not surprising forms here; they are the expected neuter singular forms.

Because it is the direct object of both infinitives.

• conferre unam rem cum alia = to compare one thing with another
• praeferre unam rem omnibus = to prefer one thing to all

So rem is accusative because it is the thing being compared or preferred.

Because the noun is res, which is feminine.

So the adjective unus, una, unum has to agree with res in gender, number, and case:

• unam rem = feminine singular accusative

A native English speaker may expect something more like one thing without thinking about gender, but in Latin the adjective must match the noun.

Latin could sometimes leave it understood, but repeating it here makes the sentence clear and neatly balanced.

The sentence is built in a very parallel way:

• comparativum unam rem cum alia conferre
• superlativum autem unam rem omnibus praeferre

That repetition helps show the contrast between comparative and superlative very clearly.

Because it follows cum, and cum takes the ablative.

So:

• cum alia = with another

More fully, you can understand it as cum alia re, meaning with another thing. The noun re is left out because Latin often omits a noun when the adjective or pronoun makes the meaning clear.

Yes, very likely re is understood.

So:

• cum alia really means cum alia re
• literally, with another thing

This kind of omission is very common in Latin. English does something similar when we say the rich, meaning rich people.

Because praeferre often takes:

• the thing preferred in the accusative
• the thing or things it is preferred to in the dative

So here:

• unam rem = the one thing being preferred
• omnibus = to all or above all others

This is a standard pattern with praeferre.

Yes, there can be.

Omnibus by itself means to all or to all things / all others, depending on context. Here the idea is something like:

• omnibus rebus = to all things
• or simply to all others

Latin often leaves the noun unspoken when the meaning is obvious enough from the context.

Autem is a contrast word, usually meaning however, but, or on the other hand.

So the sentence contrasts two ideas:

• the comparative compares one thing with another,
• but the superlative puts one thing above all.

A very important point: autem is usually postpositive, which means it tends to come second in its clause, not first. That is why Latin says superlativum autem, not autem superlativum.

Because this sentence is not actually making a normal adjective comparison such as greater than or better than.

Instead, it is talking about the action of comparing, using the verb conferre:

• conferre aliquid cum aliqua re = to compare something with something

So cum belongs naturally with the verb conferre. If this were a regular comparison with a comparative adjective, then forms like quam might come into play, but that is a different construction.

Both are compounds of ferre, meaning to carry or to bear.

• con-ferre literally = to bring together
• prae-ferre literally = to carry before

From those literal meanings, the usual senses develop:

• conferre = to compare
• praeferre = to prefer

Knowing the literal idea can make the sentence easier to remember: the comparative brings two things together, while the superlative carries one thing before all the others.

A good way is to divide it into the main verb and two parallel indirect statements:

• Lucia dicit
• comparativum unam rem cum alia conferre
• superlativum autem unam rem omnibus praeferre

So the structure is:

• Lucia says
  • that the comparative compares one thing with another
  • but that the superlative prefers one thing to all

Seeing that parallel structure makes the grammar much easier to follow.