Cum caelum serenum est, nautae e portu exeunt.

Here cum means when.

Latin cum can have several meanings depending on context:

• when / while as a conjunction
• since
• although
• with when used as a preposition

In this sentence, cum introduces a time clause: when the sky is clear.

Because this is a straightforward, factual time clause.

Latin often uses:

• cum + indicative for a simple factual when
• cum + subjunctive for more nuanced meanings like since, although, or background circumstances in past narration

Here the sentence is just stating a regular fact: when the sky is clear, the sailors leave the harbor. So est is in the indicative, not the subjunctive.

Because serenum has to agree with caelum.

• caelum = sky, neuter singular
• serenus, serena, serenum = clear, calm, serene

Since caelum is neuter singular nominative, the adjective must also be neuter singular nominative: serenum.

So:

• caelum serenum = the clear sky
• serenus caelum would be wrong, because serenus is masculine, not neuter

Also, adjective position in Latin is flexible, so both caelum serenum and serenum caelum could be possible in principle, but the agreement must stay correct.

Because it is the subject of est.

In the clause caelum serenum est:

• caelum = subject
• serenum = predicate adjective describing the subject
• est = is

So both caelum and serenum are nominative, because they refer to the same thing: the sky is clear.

Because nautae is the subject of exeunt.

• nauta = sailor
• nautae can mean either sailors (nominative plural) or of the sailor / to the sailor (genitive/dative singular), depending on context

Here the verb exeunt is plural, so nautae must mean sailors and is therefore nominative plural.

A useful note: nauta is a first-declension noun, but it refers to a male person. So although it has first-declension endings, it can still mean sailor, a masculine noun.

Portu is ablative singular.

It is used after e in the phrase e portu, which means out of the harbor or from the harbor.

The prepositions a/ab, de, e/ex, cum, in (in some meanings), and others commonly take the ablative.

So:

• portus = harbor, port
• portu = in/from/by the harbor, depending on context
• e portu = out of the harbor

Both e and ex mean out of and both can be correct.

In many cases:

• ex is used before vowels and sometimes for clarity or style
• e is often used before consonants

Since portu begins with a consonant, e portu is perfectly normal.

You may also see ex portu in Latin. The choice is often stylistic rather than a big grammatical difference.

Exeunt comes from exire, meaning to go out, to leave.

It breaks down like this:

• ex- = out
• ire = to go

The form exeunt means:

• they go out
• they leave

More specifically, it is:

• present tense
• indicative mood
• active voice
• third person plural

So it matches nautae = the sailors.

Because Latin word order is flexible.

English usually depends strongly on word order, but Latin relies much more on case endings. So Latin can move words around for emphasis, rhythm, or style.

This sentence uses a very natural order:

• subordinate clause first: Cum caelum serenum est
• main clause second: nautae e portu exeunt

The verb often does come at the end in Latin, but it does not have to.

Yes, grammatically it is a complete clause:

• subject: caelum
• adjective: serenum
• verb: est

But because it begins with cum meaning when, it functions as a subordinate clause, not the main statement of the sentence.

So it is complete in structure, but dependent in function.

In some kinds of Latin, forms of to be can occasionally be omitted, especially in poetry or very compressed style. But in a standard beginner sentence like this, est is normally expressed.

So cum caelum serenum est is the ordinary full form.

Latin often expresses ideas a bit differently from English.

English might say:

• when the weather is clear
• when the sky is clear

Latin here chooses caelum = sky/heaven. That is a very natural way to express the idea.

So even if the English translation uses weather in some contexts, the Latin is literally talking about the sky being clear.

Yes, depending on context, cum can often be translated as when or while.

• when focuses on the time at which something happens
• while emphasizes that one thing happens during another situation

In this sentence, when is probably the most natural translation, but while the sky is clear, the sailors leave the harbor is not impossible as a grammatical interpretation.

Because Classical Latin has no definite article.

English distinguishes:

• sky
• the sky

Latin usually just uses caelum, and the context tells you whether to translate it as:

• a sky
• the sky
• simply sky

The same is true for nautae and portu:

• nautae = sailors / the sailors
• portu = harbor / the harbor

English requires an article much more often than Latin does.