Pratum, in quo vaccae pascuntur, prope lacum et paludem est.

It has:

• a main clause: Pratum ... prope lacum et paludem est
• an embedded relative clause: in quo vaccae pascuntur

So the basic skeleton is:

• Pratum est = The meadow is
• prope lacum et paludem = near the lake and the marsh
• in quo vaccae pascuntur = extra information describing pratum

A useful way to see it is:

The meadow, [in which the cows graze], is near the lake and the marsh.

Because quo is ablative singular, and the preposition in here takes the ablative when it means in/on a place.

So:

• in + ablative = in/on somewhere
• in + accusative = into/onto somewhere

Here the idea is location, not motion:

• in quo = in which
• not into which

Since pratum is neuter singular, the relative pronoun must also be neuter singular in gender and number. Its case, however, is determined by its role inside the relative clause. Because it follows in of location, it becomes ablative: quo.

Because Latin relative pronouns regularly refer back to an earlier noun called the antecedent.

Here:

• pratum = antecedent
• quo = relative pronoun referring back to pratum

Latin does not need the relative pronoun to stand immediately beside its antecedent. Once you recognize the pattern, in quo naturally means in which, referring back to the meadow.

Yes, Latin can sometimes use ubi = where, but in quo is especially natural when Latin wants to connect the clause directly to a specific noun.

So:

• pratum, in quo vaccae pascuntur = the meadow, in which cows graze
• pratum, ubi vaccae pascuntur would also make sense in many contexts as the meadow where cows graze

The version with in quo makes the grammatical link to pratum especially explicit.

Here vaccae is nominative plural and is the subject of pascuntur.

So:

• vaccae pascuntur = the cows graze / are feeding

A beginner might notice that vaccae could also be genitive singular or dative singular in other contexts, but here the verb is plural, so vaccae must be nominative plural.

Because this verb can be tricky.

Pascuntur is a passive-looking form, but with animals it is commonly understood as graze, feed, or are feeding. In dictionaries and textbooks you may meet:

• pasco = I feed, pasture
• pascor / passive-type forms = I graze, feed on

So although the ending looks passive, the natural English meaning here is often active:

• vaccae pascuntur = the cows graze

This is one of those places where Latin form and best English translation do not match word-for-word.

Because prope takes the accusative.

So:

• prope lacum = near the lake
• prope paludem = near the marsh
• together: prope lacum et paludem

This is just something to memorize about prope: even though English says near without any visible case change, Latin normally uses the accusative after it.

Because palus is the dictionary form nominative singular, while paludem is the accusative singular form actually needed after prope.

The noun is third declension:

• nominative: palus = marsh
• accusative: paludem

By contrast, lacus is fourth declension:

• accusative singular: lacum

So both nouns are singular accusatives, but they form that accusative differently because they belong to different declensions.

Because Latin word order is much freer than English word order.

Latin often puts the verb at or near the end, especially in straightforward prose. So:

• Pratum ... est is perfectly normal

English usually prefers:

• The meadow is ...

Latin can move words around more freely because case endings show each word’s function. The ending of pratum, lacum, paludem, and quo helps tell you what each word is doing, so position is less important than in English.

They help the reader, but they are not the core of the grammar.

Here the commas mark off the relative clause:

• Pratum, in quo vaccae pascuntur, prope lacum et paludem est.

That shows that in quo vaccae pascuntur is extra descriptive information about pratum.

In ancient Latin writing, punctuation was much less standardized than in modern printed texts. So the commas are mainly a reading aid in modern editions.

Because the relative pronoun already does that job.

Instead of saying something like:

• the meadow, and in the meadow the cows graze

Latin uses a relative pronoun:

• pratum, in quo vaccae pascuntur

This is exactly the kind of thing relative pronouns are for: they avoid repetition and link one clause smoothly to a noun in another clause.