Aequor maris placidum est.

Aequor is nominative singular neuter. It is the subject of the sentence.

The verb est means is, so the basic structure is:

• aequor = subject
• est = verb
• placidum = predicate adjective describing the subject

So aequor is the thing being said to be calm.

Because maris is the genitive singular form of mare.

• mare = sea
• maris = of the sea

So aequor maris means something like the surface/expanse of the sea or the sea’s surface.

This is a very common Latin pattern: one noun in the genitive modifies another noun.

Because placidum agrees with aequor, and aequor is neuter singular.

Latin adjectives must agree with the noun they describe in:

• gender
• number
• case

Since aequor is nominative singular neuter, the adjective must also be nominative singular neuter:

• masculine: placidus
• feminine: placida
• neuter: placidum

So placidum is the correct form.

It describes aequor, not maris.

You can tell because of agreement:

• aequor = nominative singular neuter
• placidum = nominative singular neuter

But maris is genitive singular, so placidum cannot be agreeing with it.

In other words, the sentence says that the sea’s surface is calm, not that the sea is in the genitive somehow being described directly by placidum.

It is an adjective, but here it is used as a predicate adjective.

That means it is linked to the subject by a form of to be:

• aequor ... placidum est = the sea’s surface is calm

This is different from an adjective used directly next to a noun, such as:

• aequor placidum = the calm surface

In your sentence, placidum is not just attached to the noun inside a noun phrase; it is part of the statement being made about the subject.

Because Latin word order is much more flexible than English word order.

In English, is usually has to come between subject and complement:

• The sea’s surface is calm

In Latin, est can appear in different places, and putting it at the end is very common, especially in simple statements.

So:

• Aequor maris placidum est

is perfectly normal Latin word order.

Yes, to a large extent.

Because Latin uses case endings, the meaning does not depend as heavily on word order as English does. These could all express roughly the same basic idea:

• Aequor maris placidum est
• Placidum est aequor maris
• Maris aequor placidum est

But the order can slightly change emphasis or style. The original order is a straightforward, natural arrangement.

Because Latin does not have definite or indefinite articles like English the or a/an.

So a Latin noun like aequor can mean, depending on context:

• the surface
• a surface
• simply surface

Likewise maris can be understood as of the sea without a separate word for the.

Latin relies on context where English uses articles.

You would look them up as:

• aequor, aequoris = level surface, sea surface, expanse
• mare, maris = sea
• placidus, placida, placidum = calm, gentle, quiet
• sum, esse, fui = to be

So even though the sentence contains maris and placidum est, the dictionary entries are mare and placidus, and the verb is listed under sum.

Aequor is a third-declension neuter noun.

Its nominative singular is aequor, and its genitive singular is aequoris.

That matters because many third-declension nouns do not look very predictable from the nominative alone, so learners often need the genitive form to identify the stem:

• nominative: aequor
• genitive: aequoris
• stem: aequor- / aequor(i)-

In this sentence, though, you only need to recognize that aequor is the nominative subject.

Yes. Mare, maris is also third declension, and it is neuter.

In the sentence, you see its genitive singular form:

• mare = nominative singular
• maris = genitive singular

So this sentence contains two third-declension neuter nouns:

• aequor
• mare

Because aequor does not mean exactly the same thing as mare.

• mare = the sea
• aequor = a level surface, and often specifically the sea as a flat expanse or surface

So aequor maris is a bit more specific or literary than simply mare. It suggests the level surface of the sea or the watery expanse.

A learner will often notice that Latin has several related words where English might just use sea in all cases.

A common Classical pronunciation would be roughly:

• aequor = AI-kwor
• maris = MA-ris
• placidum est = PLA-ki-dum est

More precisely:

• ae sounds like the ai in aisle
• qu sounds like kw
• c is always hard, like k
• vowel quantity matters in formal pronunciation, though beginners often learn that later

So the whole sentence is approximately:

AI-kwor MA-ris PLA-ki-dum est

Sometimes, yes, but not always.

Latin can omit forms of sum (to be) more easily than English, especially in poetry or very compressed styles. But in ordinary prose, est is often stated.

So:

• Aequor maris placidum est = normal and complete
• Aequor maris placidum = possible in some contexts, but more compressed and less explicit

For a learner, it is best to understand the full version first.