Prope stagnum anser albus lente natat.

Because prope is a preposition meaning near, and it normally takes the accusative case in Latin.

So:

• prope = near
• stagnum = pond, in the accusative singular

A learner might expect something like a dative or ablative idea after near, but with prope, Latin uses the accusative.

We know it from both case and verb agreement.

• anser is nominative singular
• albus is also nominative singular masculine
• natat is third person singular

That means anser albus is the one doing the action: the white goose is swimming.

Meanwhile, stagnum is not the subject. It is in the accusative because it is governed by prope.

In Latin, adjective position is much freer than in English. Both of these can mean the same thing:

• anser albus
• albus anser

So albus can come after the noun without sounding strange. What matters most is that the adjective agrees with the noun in:

• gender
• number
• case

Here both anser and albus are masculine nominative singular.

Because albus has to agree with anser.

• anser is masculine
• singular
• nominative

So the adjective must also be masculine singular nominative:

• albus

If the noun were feminine, you would expect alba.
If it were neuter, you would expect album.

This is a basic Latin rule: adjectives agree with the nouns they describe.

Because lente is an adverb, not an adjective.

• lentus, lenta, lentum = slow
• lente = slowly

Here it modifies the verb natat, telling us how the goose swims.

So:

• anser albus = the white goose
• lente natat = swims slowly

If you used lentus or lenta, you would be describing a noun, not the action.

Natat is:

• third person
• singular
• present
• active
• indicative

It means he/she/it swims or is swimming.

It matches the singular subject anser albus.

The dictionary form is usually given as nato, natare, meaning to swim.

Because Latin does not have articles like English does.

So anser albus can mean:

• a white goose
• the white goose

The exact sense depends on context.

This is very normal in Latin. Learners often want to match every English word with a Latin word, but articles usually are not expressed at all.

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

For example, these could all mean essentially the same thing:

• Prope stagnum anser albus lente natat.
• Anser albus prope stagnum lente natat.
• Lente prope stagnum anser albus natat.

The differences are mostly about emphasis or style, not basic meaning.

In this sentence, putting natat at the end is very natural Latin style, since verbs often come last.

Not exactly. Stagnum is in the accusative, but it is not the direct object of natat.

Instead:

• prope is the preposition
• stagnum is the noun governed by that preposition

So prope stagnum is a prepositional phrase meaning near the pond.

The goose is not swimming the pond; it is swimming near the pond.

In dictionary form, anser, anseris is usually treated as a masculine noun meaning goose or gander. In many simple teaching sentences, it is used as a general word for goose.

That is why the adjective is masculine: albus.

So in a beginner sentence like this, the main thing to notice is the grammar:

• anser is masculine
• therefore the adjective is masculine too

Even if English just says goose, Latin still follows the grammatical gender of the noun.