Plumbum grave est.

Plumbum is nominative singular here, so it is the subject of the sentence.

A beginner might notice that plumbum could also look like accusative singular, because many neuter second-declension nouns have the same form for nominative and accusative singular. But in this sentence, est means is, and the sentence structure clearly shows plumbum as the thing being described: Lead is heavy.

So:

• plumbum = subject
• grave = predicate adjective
• est = is

Because plumbum is neuter singular, the adjective has to agree with it.

The dictionary form of the adjective is gravis, grave, meaning heavy. This is a third-declension adjective:

• masculine/feminine nominative singular: gravis
• neuter nominative singular: grave

Since plumbum is a neuter singular noun, Latin uses the neuter singular form of the adjective:

• plumbum grave est = lead is heavy

This is an example of adjective agreement in gender, number, and case.

Plumbum is a second-declension neuter noun.

That means:

• its nominative singular ends in -um
• it is grammatically neuter
• it follows the common second-declension neuter pattern

For example:

• nominative singular: plumbum
• accusative singular: plumbum
• nominative plural: plumba
• accusative plural: plumba

A useful rule to remember is:
neuter nominative and accusative are always the same, and in the plural they usually end in -a.

Est is the third-person singular present indicative active form of esse, which means to be.

So est means:

• he is
• she is
• it is

In this sentence, since plumbum is a thing, the natural English translation is it is, though in normal English we simply say:

• Lead is heavy

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

So plumbum can mean:

• lead
• the lead
• a piece of lead

Which one is best depends on context. In a simple sentence like Plumbum grave est, English usually translates it naturally as:

• Lead is heavy

So the absence of an article is completely normal in Latin.

Latin word order is more flexible than English word order because the endings show the grammatical relationships.

So all of these can mean basically the same thing:

• Plumbum grave est
• Plumbum est grave
• Grave plumbum est
• Grave est plumbum

The most neutral version here is probably Plumbum grave est or Plumbum est grave.

Even though the meaning stays the same, changing the order can change the emphasis:

• putting grave earlier may emphasize heavy
• putting plumbum earlier may emphasize lead

Here grave is an adjective, not an adverb.

It describes plumbum:

• plumbum = lead
• grave = heavy

The sentence pattern is:

• noun + adjective + est

So this is a predicate adjective construction:
Lead is heavy.

It is not modifying the verb est. It is describing what plumbum is like.

Yes. Gravis can mean more than just heavy in a literal sense.

Depending on context, it can also mean things like:

• serious
• severe
• important
• burdensome

But with plumbum (lead), the most natural meaning is the physical one:

• heavy

So here grave is understood literally.

Because with esse (to be), the adjective that describes the subject is usually in the nominative, just like the subject.

This is called a predicate adjective.

So in:

• Plumbum grave est

both of these are nominative singular neuter:

• plumbum
• grave

Latin is not saying lead sees heavy or lead carries heavy or anything like that. It is simply linking the subject to a description:

• Lead is heavy

In a common reconstructed Classical pronunciation, it would be approximately:

PLOOM-boom GRA-weh est

A few useful points:

• v is pronounced like English w
• g is always hard, as in go
• final -e in grave is pronounced
• u is a real vowel, not like English silent letters

A more careful pronunciation would avoid making the u too long in every syllable, but the approximation above is good for a beginner.

Sometimes Latin can omit a form of to be, especially in poetry or very compressed style. But in a simple basic sentence like this, est is perfectly normal and expected.

So:

• Plumbum grave est = standard, clear sentence

You may later see examples where est is understood rather than stated, but for beginning Latin, it is best to read this as a normal full sentence with an explicit verb.