Mare hodie placidum est, et nautae laeti sunt.

Because placidum has to agree with mare in gender, number, and case.

• mare = sea
• It is neuter
• It is singular
• Here it is nominative (the subject)

So the adjective must also be neuter singular nominative:

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

That is why Latin says mare placidum est = the sea is calm.

Mare is nominative singular.

It is the subject of est (is), so nominative is what we expect.

This noun belongs to the third declension, and its nominative singular form is mare. In this sentence:

• mare = subject
• est = verb
• placidum = adjective describing the subject

So the basic structure is: The sea is calm.

Because it is a predicate adjective. In Latin, an adjective after a form of esse (to be) still agrees with the noun it describes.

So in:

• mare placidum est

both mare and placidum are nominative singular neuter.

This is similar to English The sea is calm, where calm describes the subject rather than acting as an object.

Because nauta is one of the common Latin nouns that is first declension in form but masculine in meaning and grammar.

So:

• singular: nauta = sailor
• plural: nautae = sailors

Even though -a / -ae often looks feminine, some jobs or male persons in Latin are first declension but masculine, such as:

• nauta = sailor
• poeta = poet
• agricola = farmer

That is why the adjective is laeti, not laetae.

Because laeti agrees with nautae, and nautae is masculine plural here.

So the adjective must also be masculine plural nominative:

• masculine plural: laeti
• feminine plural: laetae
• neuter plural: laeta

Thus:

• nautae laeti sunt = the sailors are happy

Because the verb esse (to be) must agree with its subject in number.

• mare is singular, so Latin uses est = is
• nautae is plural, so Latin uses sunt = are

So:

• mare ... est = the sea ... is
• nautae ... sunt = the sailors ... are

Hodie means today, and it is an adverb.

Adverbs in Latin do not agree with nouns the way adjectives do, so hodie stays the same form. It tells us when the sea is calm.

So:

• Mare hodie placidum est = The sea is calm today

You could also move hodie to another place in the sentence without changing the basic meaning, for example:

• Hodie mare placidum est
• Mare placidum hodie est

Latin word order is flexible.

Latin word order is much more flexible than English because Latin shows grammatical relationships mainly through endings, not fixed position.

English usually prefers:

• The sea is calm today, and the sailors are happy.

Latin can say:

• Mare hodie placidum est, et nautae laeti sunt.

Even though the order is different, the endings show what goes together:

• mare with placidum
• nautae with laeti
• est for the singular subject
• sunt for the plural subject

So the meaning stays clear.

Because classical Latin has no definite article and no indefinite article.

So Latin does not have separate words for:

• the
• a
• an

That means:

• mare can mean sea, the sea, or sometimes a sea
• nautae can mean sailors or the sailors

You understand which is best from context.

Et means and. It connects the two parts of the sentence:

• Mare hodie placidum est
• nautae laeti sunt

So the full sentence means:

• The sea is calm today, and the sailors are happy.

Et is the ordinary Latin word for and and is very common.

Yes. Sunt means are, and laeti means happy.

Together they form a normal Latin way to say are happy:

• nautae laeti sunt = the sailors are happy

Just as in the first half:

• mare placidum est = the sea is calm

Latin often uses noun + adjective + form of esse for this kind of statement.

A common classroom pronunciation would be roughly:

• MAH-reh HOH-dee-eh PLAH-kee-doom est, et NOW-tie LYE-tee soont

A few useful points:

• mare = MAH-reh
• hodie = HOH-dee-eh
• placidum = PLAH-kee-doom
• nautae = NOW-tie in a common classical-style pronunciation
• laeti = LYE-tee in a common classical-style pronunciation
• sunt = soont

Pronunciation systems can vary a little depending on whether someone is using a classical or ecclesiastical style.

Sometimes Latin, especially in poetry or very compressed style, can omit a form of esse if the meaning is obvious. But in a normal beginner sentence like this, est and sunt are included.

So this sentence is a straightforward, complete prose sentence:

• mare ... est
• nautae ... sunt

That makes the grammar especially clear for learners.