Si tempestas in mari est, nautae magnum periculum timent.

Si means if. It introduces a conditional clause.

So the sentence is built in two parts:

• Si tempestas in mari est = If there is a storm at sea
• nautae magnum periculum timent = the sailors fear great danger

This is a very common Latin pattern: si + clause, then the main clause.

Tempestas is nominative because it is the subject of est.

In tempestas in mari est, the thing that is or exists is the storm, so tempestas must be nominative.

A very literal breakdown is:

• tempestas = storm
• in mari = in the sea / at sea
• est = is

So: A storm is at sea → more natural English: There is a storm at sea.

Latin does not need a special word like English there in expressions such as there is or there are.

So:

• tempestas est literally = a storm is
• natural English = there is a storm

The English there is just a dummy subject; Latin usually does not use one here.

Because in takes the ablative when it means in or on in the sense of location.

Here, in mari means in the sea or more naturally at sea.

Compare the basic rule:

• in + ablative = in/on (location)
• in + accusative = into/onto (motion toward)

So:

• in mari = in the sea / at sea
• in mare would mean into the sea

Mari is ablative singular of mare, maris (sea).

Mare is a third-declension neuter noun, so its ablative singular is mari.

That is why after in of location you get:

• dictionary form: mare
• in this sentence: mari

So in mari = in the sea / at sea.

Here nautae is nominative plural, meaning sailors.

We know that because it is the subject of timent, and timent is third person plural: they fear.

So:

• nautae = the sailors
• timent = fear / are fearing

Together: the sailors fear

Because nauta, nautae is a first-declension noun, even though it usually refers to a male person.

Latin has a number of nouns for jobs or roles that are:

• first declension in form
• but masculine in meaning and grammar

So nauta declines like a first-declension noun, but it means sailor and is treated as masculine.

This is normal Latin, even if it feels surprising at first.

Because magnum is an adjective agreeing with periculum.

Periculum means danger and is accusative singular neuter here, because it is the direct object of timent.

So the adjective must match it in:

• case: accusative
• number: singular
• gender: neuter

That gives:

• periculum = danger
• magnum periculum = great danger

Since periculum is neuter, the adjective is magnum, not magnus or magna.

Because timent is a transitive verb and takes a direct object in the accusative.

The sailors are fearing what?
They are fearing great danger.

So:

• nautae = subject
• timent = verb
• magnum periculum = direct object

That is why periculum is accusative singular.

Timent is:

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

Its dictionary form is timeo, timere, meaning to fear.

So timent means:

• they fear
• or they are fearing

In this sentence, they fear is the most natural translation.

Because the sentence expresses a general condition:

If there is a storm at sea, the sailors fear great danger.

This does not refer only to one specific moment. It means something like:

• whenever there is a storm at sea, the sailors fear great danger

Latin commonly uses the present indicative in both parts for this kind of straightforward condition.

Yes, the word order is perfectly normal Latin.

A very literal order is:

• If a storm at sea is, sailors great danger fear.

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

A few useful points:

• est often comes late in the clause
• the verb often comes at or near the end
• adjectives can come before or after nouns
• the important words can be placed for emphasis

So although the order is not very English-like, it is natural in Latin.

It can mean weather, season, or storm, depending on context.

In this sentence, storm is clearly the best meaning, because:

• it is in mari = at sea
• and it causes magnum periculum = great danger

So here tempestas is best understood as storm, not just weather in a neutral sense.