Pastor puellis paludem, per quam anates lente natant, ostendit.

Because puellis means to/for the girls, and with ostendere (to show) Latin often uses:

• the thing shown in the accusative
• the person shown to in the dative

So in this sentence:

• paludem = the swamp/marsh → direct object
• puellis = to the girls → indirect object

A helpful pattern is:

aliquid alicui ostendere = to show something to someone

Paludem is accusative because it is the direct object of ostendit: it is the thing the shepherd shows.

Its dictionary form is palus, paludis (feminine), meaning swamp, marsh, bog.

So the forms here are:

• nominative: palus
• accusative singular: paludem

This is a third-declension noun, which is why the accusative singular ends in -em.

Per quam means through which.

• per = through
• quam = which

Together they introduce the relative clause:

per quam anates lente natant
= through which ducks swim slowly

Here quam refers back to paludem.

Because quam is the accusative singular feminine form of the relative pronoun.

That happens for two reasons:

1. It refers back to paludem, so it must match paludem in:

   • gender: feminine
   • number: singular

2. It is the object of the preposition per, and per takes the accusative.

So:

• antecedent: paludem → feminine singular
• case required by per → accusative

Therefore the correct form is quam.

If it were quae, that would usually be nominative singular feminine, which would not fit after per.

Because anates is the subject of natant.

Inside the relative clause:

anates lente natant
= ducks swim slowly

Since ducks are doing the action of swimming, anates must be nominative plural.

Its dictionary form is anas, anatis, usually feminine.

So here:

• anates = nominative plural
• natant = they swim

Lente is an adverb meaning slowly.

It modifies natant:

• natant = they swim
• lente natant = they swim slowly

It does not describe anates or paludem; it describes the manner of the action.

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

In this sentence:

Pastor puellis paludem, per quam anates lente natant, ostendit.

Latin can place words where they are stylistically useful, not only where English would expect them. A few things to notice:

• Pastor comes first as the topic/subject.
• puellis and paludem appear before the verb.
• ostendit comes at the end, which is very common in Latin.
• the relative clause per quam anates lente natant is placed right after paludem, the noun it describes.

So although the order is different, the case endings make the structure clear.

Formally, ostendit can mean either:

• he/she shows (present)
• he/she showed (perfect)

The form is identical in ordinary spelling.

Usually the context tells you which meaning is intended. In a beginner sentence like this, it is very often meant as the present:

The shepherd shows the marsh to the girls...

So yes, a learner is right to notice the ambiguity: the form itself can be either present or perfect.

Because the relative pronoun quam clearly points back to paludem.

Here is why:

• quam is feminine singular
• paludem is feminine singular
• puellis is dative plural

So quam cannot refer to puellis. It must refer to paludem.

Also, the sense fits:

• the marsh through which ducks swim slowly

That is much more logical than trying to connect through which with the girls.

The comma helps the reader see that per quam anates lente natant is a relative clause inserted into the main sentence.

Main sentence:

• Pastor ... paludem ... ostendit
• The shepherd shows the marsh ...

Inserted description:

• per quam anates lente natant
• through which ducks swim slowly

In modern printed Latin, punctuation is used for clarity, much as in English. Ancient Latin texts originally had much less punctuation, so the grammar itself is what really matters.