Quadrans adulterinus est.

• quadrans is the subject: the thing being talked about.
• adulterinus is a predicate adjective: it describes the subject after a form of to be.
• est is the verb is, from sum, esse.

So the structure is:

subject + predicate adjective + linking verb

That is a very common Latin pattern.

Because est is a linking verb, not an action verb.

With a linking verb like sum (to be), Latin puts the subject in the nominative, and a predicate noun or predicate adjective that refers back to the subject is also in the nominative.

So here:

• quadrans = nominative subject
• adulterinus = nominative adjective describing that subject

This is similar to how Latin says things like:

• Puella laeta est = The girl is happy
• Marcus medicus est = Marcus is a doctor

Because it agrees with quadrans.

Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

Here, quadrans is:

• masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives adulterinus.

If the noun were feminine, you would expect adulterina.
If it were plural masculine, you would expect adulterini, and so on.

Because Latin does not have articles like English a, an, and the.

So quadrans can mean, depending on context:

• a quadrans
• the quadrans

Latin leaves that distinction to context.

This is one of the most common adjustments for English speakers learning Latin: Latin often says less explicitly than English does in this area.

Yes. It is perfectly normal.

Latin word order is more flexible than English because endings show how words function. In a simple sentence like this, several orders are possible:

• Quadrans adulterinus est
• Quadrans est adulterinus
• Adulterinus quadrans est

These all express basically the same core idea, but the emphasis can shift a little.

The version Quadrans adulterinus est is natural and straightforward.

Sometimes, yes, but usually not in a plain beginner-style prose sentence like this.

Latin can omit forms of sum when the meaning is obvious, especially in:

• poetry
• inscriptions
• very compressed style
• certain rhetorical contexts

So you might occasionally see something like Quadrans adulterinus, with est understood.

But in ordinary prose, est is completely normal and usually clearer for learners.

Quadrans is a noun of the third declension.

Its basic dictionary form is:

• quadrans, quadrantis (masculine)

Historically, it can refer to a quarter or, in Roman usage, a small coin worth a quarter of an as.

So in a sentence like this, the cultural context is often monetary: it is talking about a coin.

Not here.

That is a very easy mistake for English speakers to make because the words look related.

In this sentence, adulterinus means something like:

• counterfeit
• spurious
• adulterated
• not genuine

So it describes the quadrans as false or tampered with, not sexually immoral.

The English word adulterate is actually a better clue here than adulterous.

A good vocabulary-style listing would be:

• quadrans, quadrantis m. = a quarter; a small coin
• adulterinus, adulterina, adulterinum = counterfeit, spurious, adulterated
• sum, esse, fui, futurus = to be

A few useful notes:

• quadrans is a third-declension noun
• adulterinus is a first/second-declension adjective
• est is the 3rd person singular present active indicative of sum

In a restored Classical pronunciation, roughly:

KWAH-drahns ah-dool-teh-REE-noos est

A few points:

• qu is pronounced like kw
• c and g are always hard in Classical Latin, though this sentence has neither
• adulterinus has the stress on ri: adulterīnus
• est is pronounced like est, not like English is

You do not need perfect pronunciation to understand the grammar, but it helps to notice the word shapes and endings clearly.