Avus dicit aquam apud fontem purissimam esse.

Because Latin often uses an accusative + infinitive construction after verbs of saying, thinking, knowing, and perceiving.

In this sentence:

• Avus dicit = Grandfather says
• aquam ... esse = that the water is ...

Inside that reported statement, aquam is the logical subject of esse, but Latin puts that subject in the accusative rather than the nominative.

So:

• aqua est purissima = the water is very pure / purest
• avus dicit aquam esse purissimam = grandfather says that the water is very pure / purest

This is one of the most important sentence patterns in Latin.

Because after dicit, Latin normally uses an infinitive clause for indirect statement.

So instead of saying something like:

• Avus dicit quod aqua ... est

Classical Latin much more typically says:

• Avus dicit aquam ... esse

Here esse means to be, and together with the accusative subject it forms the reported statement:

• aquam ... esse = that the water is ...

English usually uses that plus a normal verb, but Latin usually prefers the accusative-and-infinitive structure.

Because purissimam goes with aquam.

It is a predicate adjective in the infinitive clause, and it must agree with aquam in:

• gender: feminine
• number: singular
• case: accusative

So:

• aquam = feminine singular accusative
• purissimam = feminine singular accusative

Even though purissimam is describing the water, it is not directly attached like puram aquam would be. Instead, it is part of the statement aquam ... purissimam esse = that the water is very pure / purest.

It describes aquam, not fontem.

You can tell because of agreement:

• aquam = feminine singular accusative
• purissimam = feminine singular accusative

But:

• fontem = masculine singular accusative

If purissimam described fontem, it would have to be masculine, something like purissimum? No—because fons is masculine, accusative singular would actually be purissimum fontem only if the adjective were neuter? Let's be careful: masculine accusative singular of purissimus is purissimum. So yes, it would need to be masculine accusative singular, not purissimam.

So the grammar makes it clear that the water is what is being called pure.

Apud fontem means near the spring/fountain or by the spring/fountain.

• apud is a preposition meaning near, at, by, sometimes among depending on context.
• fontem is accusative because apud takes the accusative.

So aquam apud fontem means the water near the spring.

In this sentence, apud fontem tells you which water is being discussed.

Because apud always takes the accusative.

So:

• apud fontem = near the spring
• fontem is the accusative singular of fons, fontis

This accusative is required by the preposition, not because fontem is the direct object of dicit.

Because apud means near/by/at the side of, while in usually means in or on, depending on context.

So:

• apud fontem = near the spring
• in fonte would mean in the spring
• ad fontem would often mean to the spring or sometimes at the spring, depending on context

A learner should notice that Latin is being quite specific about location:

• apud = nearby, in the vicinity of
• not necessarily literally inside the water source

Latin word order is much more flexible than English word order because Latin shows grammatical relationships mainly through endings, not position.

So in:

• Avus dicit aquam apud fontem purissimam esse

you identify the structure by forms, not by order:

• Avus = nominative subject
• dicit = main verb
• aquam ... esse = indirect statement
• purissimam agrees with aquam
• apud fontem is a prepositional phrase

English depends much more on word order, but Latin can move words around for emphasis, rhythm, or style.

Yes, to a large extent.

For example, these would still express basically the same idea:

• Avus aquam apud fontem purissimam esse dicit.
• Aquam apud fontem avus purissimam esse dicit.
• Avus dicit purissimam esse aquam apud fontem.

Some versions may sound more natural than others, and different word orders can shift emphasis, but the core grammar stays the same because the endings make the relationships clear.

Avus is nominative singular because it is the subject of dicit.

• avus = grandfather
• dicit = says

So Avus dicit means Grandfather says.

It is not part of the infinitive clause. The infinitive clause begins with aquam and continues through esse.

It can mean either, depending on context.

Purissimam is the superlative form of purus:

• purus = pure
• purior = purer
• purissimus/purissima/purissimum = purest, very pure

In Latin, the superlative can be:

• true superlative: the purest
• intensive: very pure

So aquam ... purissimam esse may mean:

• that the water is the purest
• or that the water is very pure

The context usually tells you which is meant.

There is no separate word for that here.

English says:

• Grandfather says that the water ... is ...

Latin usually does not use a separate that in this construction. Instead, it uses the accusative + infinitive:

• aquam ... esse

That whole phrase functions like English that the water is ...

So in Latin, the idea of that is built into the grammatical structure rather than expressed by a separate word.

A good way is to look for:

1. a verb of saying, thinking, knowing, etc.
2. an accusative noun that could be the subject of a statement
3. an infinitive

Here:

• dicit = verb of saying
• aquam = accusative subject of the reported statement
• esse = infinitive

So the indirect statement is:

• aquam apud fontem purissimam esse

Everything there belongs together as the content of what grandfather says.

Yes.

Latin has no definite article like English the and no indefinite article like a/an. So aquam can mean:

• water
• the water
• sometimes even some water

The context tells you which is most natural.

Here, because apud fontem narrows it down, English will usually translate it as the water near the spring.

They are:

• avus, -i = grandfather
• dico, dicere, dixi, dictum = say, tell
• aqua, -ae = water
• apud = near, at, among
• fons, fontis = spring, fountain
• purus, -a, -um = pure
• sum, esse, fui = be

Recognizing dictionary forms is especially helpful here because some surface forms are not obvious:

• fontem comes from fons
• purissimam comes from purus
• esse comes from sum