Sardina in aestuario natat.

Each word has a clear job:

• Sardina: the subject, sardine
• in aestuario: a prepositional phrase, in the estuary
• natat: the verb, swims

So grammatically, the sentence is:

• subject: sardina
• prepositional phrase: in aestuario
• verb: natat

We know from its case and from the overall structure of the sentence.

Sardina is nominative singular, which is the case normally used for the subject of a sentence. In a first-declension noun like this one, nominative singular often ends in -a.

Also, natat is singular, so it needs a singular subject, and sardina fits that role.

English relies heavily on word order to show who is doing the action. Latin relies much more on endings.

Because after in meaning in / inside / on, Latin usually uses the ablative case to show location.

The dictionary form is aestuarium, but in this sentence it changes to aestuario because it is ablative singular:

• nominative/accusative singular: aestuarium
• ablative singular: aestuario

So in aestuario means in the estuary.

Yes. This is a very important point.

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

So:

• in aestuario natat = it swims in the estuary
• in aestuarium natat could mean it swims into the estuary

The difference is not the preposition alone, but the case after it.

Natat is the 3rd person singular present active indicative of natare, meaning to swim.

So natat means:

• he swims
• she swims
• it swims

In this sentence, since the subject is sardina, the natural English translation is it swims or simply swims with the sardine as subject.

The ending -t is a very common sign of 3rd person singular in Latin verbs.

It can mean either, depending on context.

Latin does not usually make the same sharp distinction that English makes between:

• swims (simple present)
• is swimming (present progressive)

So natat can cover both ideas. In a simple teaching sentence like this, English usually gives swims, but is swimming could also be reasonable in the right context.

Because Latin does not have articles like English does.

So sardina can mean:

• a sardine
• the sardine

And aestuario in context can mean:

• in an estuary
• in the estuary

English has to choose an article, but Latin usually leaves that idea to context.

Sardina is grammatically feminine, yes, but that does not necessarily mean the fish is biologically female.

In Latin, every noun has a grammatical gender:

• masculine
• feminine
• neuter

This is a grammar category, not always a statement about real-world sex.

What it does affect is agreement. For example, an adjective describing sardina would also have to be feminine:

• sardina parva = the small sardine

No. Latin word order is much more flexible than English word order because the endings carry so much information.

Sardina in aestuario natat is perfectly good Latin, but other orders are also possible, such as:

• In aestuario sardina natat
• Natat sardina in aestuario

These can sound slightly different in emphasis, but the basic meaning stays the same.

English speakers often expect word order to tell them everything. In Latin, case endings and verb endings are usually more important.

Yes, if the subject were already clear from context.

Because natat already tells you the verb is 3rd person singular, Latin can often omit an expressed subject. So:

• in aestuario natat = he/she/it swims in the estuary

If you want to identify exactly what is swimming, then adding sardina makes that explicit.

This is one of the big differences from English: Latin often does not need a separate subject pronoun when the verb ending already gives enough information.