Puer dicit aquam nimis frigidam esse.

Because after dicit in this kind of construction, Latin often uses an accusative + infinitive structure for indirect statement.

So instead of saying something literally like The boy says that the water is too cold, Latin says more literally:

The boy says the water to be too cold.

In that pattern:

• aquam = the subject of the infinitive esse
• therefore it goes into the accusative
• esse = to be

So aquam is not the direct object of dicit in the ordinary English sense; it is the subject of the infinitive clause.

Because frigidam agrees with aquam.

In Latin, adjectives must match the nouns they describe in:

• gender
• number
• case

Here:

• aquam is feminine singular accusative
• so frigidam must also be feminine singular accusative

That is why it is frigidam, not frigida.

Because this is an indirect statement after dicit.

In English we say:

• The boy says that the water is too cold.

In Latin, a very common way to express this is:

• The boy says the water to be too cold.

That is why Latin uses:

• aquam = accusative subject
• esse = infinitive verb

So esse is required by the indirect statement construction.

There usually is no separate word for that in this construction.

English often uses that:

• The boy says that the water is too cold.

Latin often expresses the same idea simply with the accusative + infinitive:

• Puer dicit aquam nimis frigidam esse.

So the idea of that is built into the grammar, not translated by a separate word.

Nimis means too much or, with an adjective, simply too.

Here it modifies frigidam, so:

• nimis frigidam = too cold

It is an adverb, so it does not change form.

Compare:

• nimis calida = too hot
• nimis longa = too long
• nimis lente = too slowly

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

Placing esse at the end is very natural in Latin, especially in an indirect statement. Latin often likes to put the verb at or near the end of a clause.

So:

• Puer dicit aquam nimis frigidam esse

is a normal order.

But Latin can sometimes move words around for emphasis. The endings, not just the position, tell you how the words function.

Because puer is in the nominative singular, which marks it as the subject of dicit.

So:

• puer = the boy
• dicit = says

Meanwhile:

• aquam is accusative, so it belongs to the indirect statement as the subject of esse

That gives us two different layers:

1. Main clause: puer dicit = the boy says
2. Indirect statement: aquam nimis frigidam esse = that the water is too cold

Yes. It is the indirect statement, and it functions as the content of what the boy says.

Inside that unit:

• aquam = subject of the infinitive
• frigidam = predicate adjective describing aquam
• esse = infinitive verb
• nimis = adverb modifying frigidam

So the whole group means:

• the water to be too cold
• or more naturally in English, that the water is too cold

Because that would mix two different constructions.

Latin normally does not use a finite clause with est after dicit in the ordinary way English uses that ... is. Instead, classical Latin strongly prefers the accusative + infinitive for indirect statement.

So instead of:

• The boy says that the water is too cold
• with a separate clause using is

Latin usually says:

• Puer dicit aquam nimis frigidam esse

That is the idiomatic pattern learners need to get used to.

Only up to a point.

A very literal translation would be:

• The boy says the water to be too cold.

That helps show the grammar, but it is not natural English.

A natural English translation is:

• The boy says that the water is too cold.

When learning Latin, it is often useful to know both:

• the literal translation, to understand the structure
• the natural translation, to understand the meaning

Dicit is:

• 3rd person singular
• present tense
• active voice
• from dicere = to say

So it means:

• he says
• she says
• it says

Because the subject here is puer, we understand it as:

• the boy says

Then the main verb would change to plural:

• Pueri dicunt aquam nimis frigidam esse.

Changes:

• puer → pueri = the boys
• dicit → dicunt = say

The indirect statement stays the same unless its own subject or description also changes.

You would simply remove nimis:

• Puer dicit aquam frigidam esse.

That means:

• The boy says that the water is cold.

So nimis adds the idea of too.

In a common classroom pronunciation based on classical Latin:

• aquam ≈ AH-kwam
• frigidam ≈ FRI-gi-dam, with hard g as in go

A rough pronunciation of the whole sentence is:

• POO-er DEE-kit AH-kwam NEE-mis FREE-gi-dam ES-se

Pronunciation systems vary somewhat, but the grammar stays the same.