Molitor dicit se molam veterem lente vertere, sed molam novam facilius vertere.

Because Latin uses se as the reflexive pronoun, meaning himself / herself / themselves when it refers back to the subject of the main verb.

Here, the main verb is dicit = says, and its subject is molitor = the miller. So se means himself, referring back to molitor.

So:

• Molitor dicit se... vertere = The miller says that he... turns

Latin does not use se exactly the way English uses he here. In this construction, se is the normal word because it points back to the subject of dicit.

Because after verbs of saying, thinking, knowing, and perceiving, Latin often uses indirect statement.

The standard pattern is:

• verb of saying/thinking
  • accusative subject
    • infinitive

So in this sentence:

• dicit = he says
• se = accusative subject of the infinitive
• vertere = infinitive, to turn

This is the Latin way of saying:

• The miller says that he turns...

So se vertere literally means something like him to turn, but in natural English it becomes that he turns.

In an indirect statement, the subject of the infinitive goes into the accusative case.

That is why Latin has:

• se vertere rather than something nominative like is vertit

So although se is logically the subject of vertere, grammatically it is accusative because that is how the indirect statement works.

Yes. Se is understood with both infinitives.

The sentence is compressed. Expanded, it would be:

• Molitor dicit se molam veterem lente vertere, sed se molam novam facilius vertere.

Latin often leaves out a repeated word when it is easy to understand from context. So the second se is omitted because it would be repetitive.

That means the sense is:

• The miller says that he turns the old millstone slowly, but turns the new millstone more easily.

Because molam is the direct object of vertere, and veterem and novam are adjectives describing molam.

So:

• molam = direct object, therefore accusative
• veterem agrees with molam
• novam agrees with molam

They are all:

• feminine
• singular
• accusative

Agreement is very important in Latin: adjectives must match the nouns they describe in gender, number, and case.

Because vetus is the dictionary form, or nominative form. Here the adjective has to match molam, which is feminine singular accusative.

The adjective vetus declines like this in the singular:

• nominative: vetus
• accusative masculine/feminine: veterem
• genitive: veteris
• and so on

So:

• mola vetus = an old millstone as subject
• molam veterem = an old millstone as object

In this sentence, it is the object of vertere, so veterem is required.

For the same reason: the adjective must agree with molam.

• nova is nominative feminine singular
• novam is accusative feminine singular

Since molam is accusative feminine singular, the adjective must also be accusative feminine singular:

• molam novam = the new millstone

Lente is an adverb meaning slowly. It modifies vertere.

So:

• molam veterem lente vertere = to turn the old millstone slowly

A common beginner mistake is to expect an adjective here, but Latin needs an adverb because it is describing how the action is done, not describing the noun.

Here facilius is an adverb, specifically a comparative adverb meaning more easily.

It modifies vertere:

• molam novam facilius vertere = to turn the new millstone more easily

This is not describing the millstone. It is describing the manner of turning.

A useful pattern:

• facile = easily
• facilius = more easily

Comparative adverbs in Latin often end in -ius.

It compares the two actions:

• turning the old millstone slowly
• turning the new millstone more easily

The idea is that the new millstone is easier to turn than the old one.

Latin does not always state every part of the comparison explicitly. The contrast is made clear by:

• sed = but
• facilius = more easily

So the comparison is understood from the sentence as a whole.

Sed means but, and it contrasts the two infinitive phrases:

• se molam veterem lente vertere
• molam novam facilius vertere

So it sets up a contrast between:

• the old millstone
• the new millstone

and also between:

• lente = slowly
• facilius = more easily

In this context, mola most naturally means millstone, though in some contexts it can refer more broadly to a mill or grinding stone.

Because the subject is molitor = miller, and the verb is vertere = to turn, millstone is the most likely sense here.

So:

• molam veterem = the old millstone
• molam novam = the new millstone

Latin word order is much freer than English word order because the endings show the grammatical relationships.

English depends heavily on position:

• The miller says that he turns the old millstone slowly...

Latin can move words around more flexibly:

• Molitor dicit se molam veterem lente vertere...

This word order helps highlight the ideas in a natural Latin way:

• Molitor introduces the subject
• dicit gives the main action
• se... vertere sets up the indirect statement
• sed introduces the contrast
• facilius is placed near the second vertere to emphasize the easier action

So the order may feel unusual to an English speaker, but it is normal Latin.