Pumex durus est.

Because pumex is in the nominative singular, which is the case normally used for the subject of a sentence in Latin.

Here, pumex means pumice or pumice stone, and it is the thing being described as hard.

Durus is an adjective, meaning hard.

In this sentence, it describes pumex, so it works as a predicate adjective:
Pumex durus est = Pumice is hard.

Latin often uses:

• a noun
• an adjective describing that noun
• the verb to be

Because Latin adjectives must agree with the noun they describe in gender, number, and case.

• pumex is masculine
• singular
• nominative

So the adjective must also be:

• masculine
• singular
• nominative

That gives durus.

If the noun were feminine, you would expect dura.
If it were neuter, you would expect durum.

Est means is. It is the 3rd person singular form of esse, the Latin verb to be.

So:

• sum = I am
• es = you are
• est = he/she/it is

In this sentence, est links the subject pumex with the description durus.

Latin often does not need to state the subject pronoun if the verb already shows the person and number.

Since est already means he/she/it is, Latin does not need a separate word for it.

So Latin can simply say:

Pumex durus est
rather than something like Pumex id durus est, which would be unnecessary and unnatural here.

Latin has no articles like English a, an, or the.

So pumex can mean:

• a pumice stone
• the pumice stone
• sometimes just pumice

The exact sense depends on context. In a simple teaching sentence like this, English often translates it as Pumice is hard.

Yes. Latin word order is more flexible than English word order because the endings give a lot of grammatical information.

You could also see:

• Pumex est durus
• Durus est pumex
• Durus pumex est

These all can mean roughly the same thing, though the emphasis may change.

The version Pumex durus est is a straightforward, natural order.

It is also nominative singular masculine.

Even though durus is not the subject, it still appears in the nominative because it is linked to the subject by est. This is normal with predicate adjectives in Latin.

So both words match:

• pumex — nominative singular masculine
• durus — nominative singular masculine

Pumex is a third-declension noun.

Its dictionary form is usually given as:

pumex, pumicis — pumice stone

That second form, pumicis, shows the stem pumic-, which appears in other cases.

For example:

• nominative singular: pumex
• genitive singular: pumicis

No. Durus is a first-and-second-declension adjective.

Its basic forms are:

• durus — masculine
• dura — feminine
• durum — neuter

Even though pumex is third declension, the adjective does not have to belong to the same declension. It only has to agree in gender, number, and case.

A common classroom pronunciation would be roughly:

POO-meks DOO-roos est

More traditionally reconstructed, u sounds like oo, and x sounds like ks.

So:

• pumex ≈ POO-meks
• durus ≈ DOO-roos
• est ≈ est

The exact pronunciation can vary a little depending on whether you are using Classical or Ecclesiastical pronunciation.

It could do either, depending on context.

Without articles, Latin often leaves that open. So Pumex durus est might mean:

• Pumice is hard as a general statement, or
• The pumice stone is hard in a specific context

In an introductory sentence like this, it is usually understood as a general statement.