Cetus in mari natat.

Because cetus is the subject: it is the thing doing the action.

In this sentence:

• cetus = the whale
• natat = swims
• in mari = in the sea

Latin word order is often more flexible than English word order because the endings help show each word’s role. So Cetus in mari natat is a perfectly normal way to say it, but other orders are also possible, such as In mari cetus natat.

Cetus is in the nominative singular.

The nominative case is usually used for the subject of the sentence. Since the whale is the one swimming, cetus must be nominative.

So:

• cetus = nominative singular
• function = subject of natat

Because mare changes its form after the preposition in when in means in or on in the sense of location.

Here is the idea:

• dictionary form: mare = sea
• in this sentence: in mari = in the sea

That happens because in with the meaning in/on takes the ablative case, and the ablative singular of mare is mari.

So:

• nominative: mare
• ablative: mari

The preposition in can take two different cases in Latin, depending on the meaning:

• in + ablative = in/on somewhere, showing location
• in + accusative = into/onto somewhere, showing motion toward

In Cetus in mari natat, the whale is already in the sea, so this is location, not movement into it. That is why Latin uses in mari.

Natat is a verb in the present tense, third person singular.

That means:

• present tense = swims / is swimming
• third person = he, she, it
• singular = one subject

Since cetus is singular, the verb is singular too: natat = it swims or the whale swims.

It can mean either one, depending on context.

Latin present tense often covers both:

• swims
• is swimming

So Cetus in mari natat could be understood as either:

• The whale swims in the sea
• The whale is swimming in the sea

English makes a sharper distinction than Latin does here.

There is no separate word for the in this sentence.

Latin does not have articles like English the or a/an. So a word like cetus can mean:

• a whale
• the whale

And mare/mari can mean:

• a sea
• the sea

You figure out which English article makes sense from the context or the translation you have been given.

Yes. Latin allows much freer word order than English.

Because the endings show the grammar, these can all mean essentially the same thing:

• Cetus in mari natat
• In mari cetus natat
• Natat cetus in mari

However, changing the order can change the emphasis:

• first position often gives a word more prominence
• final position can also sound emphatic, especially for the verb

So the order is flexible, but not meaningless.

Mare is a third-declension neuter noun.

Its basic meaning is sea. One important form here is:

• nominative singular: mare
• ablative singular: mari

A learner often notices that mari looks unusual compared with first- or second-declension nouns, but it is a normal form for this type of third-declension noun.

In Latin, cetus is treated as a masculine noun grammatically.

That does not necessarily mean the animal is biologically male. In Latin, grammatical gender and real-life sex are not always the same thing. Here, masculine gender mainly matters for agreement with adjectives or pronouns if they are added.

A common classroom pronunciation would be something like:

KAY-toos in MAH-ree NAH-taht

More carefully:

• c in classical Latin is always hard, like k
• ce- in cetus is pronounced ke-
• mari has two syllables after the first: ma-ri
• natat is na-tat

So cetus does not sound like English see-tus.

Yes. In mari cetus natat is perfectly possible Latin.

Putting in mari first might emphasize the setting:

• In the sea, the whale swims

Latin often places words in the order that best highlights what the speaker wants to stress, rather than following a fixed English-style pattern like subject-verb-object/place every time.