Ex omnibus discipulis Lucia diligentissima est, sed Marcus celerrime scribit.

Because ex means out of or from among, and it is a very common way to express the group within which a superlative is true.

So:

• ex = from, out of, from among
• omnibus discipulis = all the students in the ablative plural

Together, ex omnibus discipulis means from among all the students or more naturally of all the students / among all the students.

This fits the idea of Lucia is the most diligent among all the students.

They are in the ablative because ex takes the ablative case.

Here is the breakdown:

• omnibus = ablative plural of omnis (all, every)
• discipulis = ablative plural of discipulus (student)

So after ex, Latin requires:

• ex omnibus discipulis

not nominative or accusative.

Yes. Latin often uses either:

• ex/de + ablative
• or the partitive genitive

So another possible way to express the same idea would be:

• Lucia omnium discipulorum diligentissima est.

That also means Lucia is the most diligent of all the students.

Both are good Latin. This sentence simply chooses ex + ablative.

Because it agrees with Lucia, which is feminine singular.

Diligentissima is an adjective, and Latin adjectives must agree with the noun they describe in:

• gender
• number
• case

Since Lucia is:

• feminine
• singular
• nominative

the adjective must also be:

• feminine
• singular
• nominative

So:

• masculine: diligentissimus
• feminine: diligentissima
• neuter: diligentissimum

It is the superlative form of diligens.

The adjective goes like this:

• positive: diligens = diligent
• comparative: diligentior = more diligent
• superlative: diligentissimus, -a, -um = most diligent

In this sentence, because Lucia is being compared with all the students, diligentissima clearly means most diligent.

A superlative can sometimes also mean very diligent, but here the comparison phrase ex omnibus discipulis shows that it is a true superlative: the most diligent.

Est is the 3rd person singular present of esse, meaning is.

So:

• Lucia diligentissima est = Lucia is the most diligent

Latin can sometimes omit forms of esse, especially in poetry or in certain compressed styles, but in normal prose it is very common to include it. Here est makes the sentence complete and straightforward.

Because celerrime modifies the verb scribit.

• Adjectives describe nouns.
• Adverbs describe verbs.

Here the idea is not Marcus is the fastest person, but Marcus writes the fastest.

So Latin uses the adverb:

• celerrime = most quickly / very quickly

If you used celerrimus, that would be an adjective and would need to describe a noun, for example:

• Marcus celerrimus est = Marcus is the fastest

That is a different statement.

It comes from celer, celeris, celere = quick, swift.

This adjective forms its superlative a little differently:

• positive: celer
• comparative: celerior
• superlative: celerrimus

Then the superlative adverb is formed from the superlative stem:

• celerrimus → celerrime

So celerrime means most quickly or sometimes very quickly, depending on context.

Scribit is:

• 3rd person
• singular
• present
• active
• indicative

from the verb scribere = to write

So it means:

• he writes
• or, depending on context, he is writing

Because the subject is Marcus, singular, the verb is also singular.

Because the two clauses are built differently.

In the first clause:

• Lucia diligentissima est

This uses a linking verb (est) plus an adjective describing Lucia.

In the second clause:

• Marcus celerrime scribit

This uses an action verb (scribit) plus an adverb describing how Marcus writes.

So:

• diligentissima tells us what Lucia is like
• celerrime tells us how Marcus writes

That is a very common difference in both Latin and English.

Latin word order is much freer than English word order because Latin shows grammatical relationships through endings, not mainly through position.

So this order:

• Ex omnibus discipulis Lucia diligentissima est

is perfectly natural. It puts ex omnibus discipulis first to set the frame: among all the students...

Likewise, putting est at the end is very common in Latin.

The sentence could be rearranged in other ways and still mean basically the same thing, though the emphasis would change. For example:

• Lucia ex omnibus discipulis diligentissima est
• Marcus sed celerrime scribit would be much less natural, because sed normally introduces the contrast at the start of the second clause

So the given order is natural and emphasizes the comparison first, then the person, then the description.

Yes. Sed is the ordinary Latin word for but.

It joins the two clauses and shows a contrast:

• Lucia is the most diligent
• but Marcus writes the fastest

So the sentence is contrasting two different strengths:

• Lucia stands out for diligence
• Marcus stands out for speed in writing