Mercatrix dicit pruna hodie recentiora esse quam heri.

Because after dicit (says), Latin often uses an indirect statement construction:

• dicit = she says
• then the reported statement goes into accusative + infinitive
• so English the plums are fresher becomes Latin pruna recentiora esse

If this were a direct statement, you would expect:

• Pruna hodie recentiora sunt quam heri.
  The plums are fresher today than yesterday.

After dicit, sunt changes to esse.

In an indirect statement, the subject is usually put in the accusative case. So you might expect prunas if you are thinking of a first-declension plural.

But pruna is a neuter plural noun, and for neuter plural nouns the nominative and accusative forms are the same.

So here:

• pruna is functioning as the subject of the infinitive esse
• grammatically, it is accusative in the indirect statement
• but its form looks the same as the nominative

That is a very common thing in Latin with neuter nouns.

Recentiora is the comparative adjective agreeing with pruna.

Here is what is happening:

• base adjective: recens = fresh
• comparative: recentior / recentius = fresher
• neuter plural nominative/accusative: recentiora

Since pruna is neuter plural, the adjective must match it:

• pruna recentiora = fresher plums

Because this is inside an indirect statement, recentiora goes with pruna and works with esse:

• pruna recentiora esse = that the plums are fresher

Quam is the normal word used after a comparative to mean than.

So:

• recentiora ... quam heri = fresher ... than yesterday

Latin is being slightly compact here. The full idea is something like:

• fresher today than they were yesterday

But Latin often leaves out repeated words when they are obvious. So after quam, it is enough just to say heri (yesterday).

They are both adverbs of time.

• hodie = today
• heri = yesterday

They do not need prepositions here. Latin often uses simple adverbs for time expressions like this.

So:

• hodie tells you when the plums are fresher
• heri gives the point of comparison

Mercatrix is nominative singular, because it is the subject of dicit.

So the main clause is:

• Mercatrix dicit = The merchant/shopkeeper says

The ending -trix is a feminine noun ending, often corresponding to a masculine noun in -tor. For example:

• mercator = male merchant
• mercatrix = female merchant

It has two parts:

1. Main clause

   • Mercatrix dicit
   • The merchant says

2. Indirect statement

   • pruna hodie recentiora esse quam heri
   • that the plums are fresher today than yesterday

So the sentence is built like this:

• [subject] + [main verb] + [reported statement]

This is a very common Latin pattern with verbs like:

• dico = say
• puto = think
• scio = know
• audio = hear

Because Latin usually does not use a conjunction equivalent to English that after verbs like say in this kind of sentence.

English says:

• The merchant says that the plums are fresher today than yesterday.

Latin usually prefers:

• The merchant says the plums to be fresher today than yesterday.

That sounds unnatural in English, but it is the normal Latin structure. The idea of that is built into the accusative + infinitive construction.

Latin word order is much more flexible than English word order because Latin endings show how words function.

English depends heavily on order:

• The merchant says the plums are fresher today than yesterday.

Latin can move things around more freely:

• Mercatrix dicit pruna hodie recentiora esse quam heri.

This order puts the main verb dicit early, and then gives the reported statement.

You could rearrange parts of the indirect statement without changing the basic meaning, as long as the forms still make the grammar clear.

Yes. A very useful way to understand it is to imagine the direct version first:

• Pruna hodie recentiora sunt quam heri.
• The plums are fresher today than yesterday.

Then after Mercatrix dicit:

• Pruna stays as the subject of the reported statement, but in indirect statement usage it is accusative in function
• sunt changes to esse

So:

• direct: pruna ... sunt
• indirect after dicit: pruna ... esse

That is a helpful pattern to remember when reading Latin.