Aqua pura infanti necessaria est.

Because both adjectives agree with aqua in gender, number, and case.

• aqua = feminine, singular, nominative
• pura = feminine, singular, nominative
• necessaria = feminine, singular, nominative

In Latin, adjectives change their endings to match the noun they describe or refer to. Since aqua is feminine singular nominative, the adjectives connected with it must also be feminine singular nominative.

Because infanti is in the dative case, not the nominative.

Here the idea is necessary for the child/infant. Latin often uses the dative after adjectives like necessarius to show the person something is necessary for or to.

So:

• aqua pura = pure water
• necessaria est = is necessary
• infanti = for the child / to the infant

This is a very common Latin pattern:

• mihi necessarium est = it is necessary for me
• nobis utile est = it is useful for us

You know because aqua is in the nominative case, which is the case normally used for the subject.

In this sentence:

• aqua = nominative singular
• infanti = dative singular

So aqua is the thing being talked about: pure water.
The sentence is saying something about it: that it is necessary for the child.

Not quite.

• pura is an attributive adjective: it directly describes what kind of water it is — pure water.
• necessaria is a predicate adjective: it tells you something about the subject through est — the water is necessary.

So the structure is:

• aqua pura = pure water
• necessaria est = is necessary

In English, the difference is similar to:

• pure water
• the water is necessary

Both adjectives agree with aqua, but they are doing slightly different jobs in the sentence.

Latin word order is much freer than English word order because the endings show the grammar.

So Aqua pura infanti necessaria est is natural Latin, but other orders are also possible, such as:

• Aqua pura necessaria est infanti
• Infanti aqua pura necessaria est

The basic meaning stays the same because the forms tell you what each word is doing.
Putting est at the end is very common in Latin.

Yes, often they could.

Because Latin relies heavily on case endings, word order is used more for emphasis and style than for basic grammar.

For example, these all mean essentially the same thing:

• Aqua pura infanti necessaria est
• Infanti aqua pura necessaria est
• Necessaria est aqua pura infanti

However, the emphasis may shift:

• starting with infanti can highlight for the child
• starting with aqua pura can highlight pure water

Because Latin has no articles.

Latin does not have separate words for the or a/an. Whether you translate aqua as water, the water, or some water depends on context.

So aqua pura could be understood as:

• pure water
• the pure water

In most beginner translations, pure water is the most natural choice.

It is the dative singular of infans, infantis.

This noun is third declension. Its forms include:

• nominative singular: infans
• genitive singular: infantis
• dative singular: infanti
• accusative singular: infantem

So in this sentence, infanti means for the child / for the infant.

Here is the breakdown:

• aqua — first declension noun
• pura — first/second declension adjective, feminine nominative singular here
• infanti — third declension noun form, dative singular of infans
• necessaria — first/second declension adjective, feminine nominative singular here
• est — third person singular present of esse, meaning is

This is a useful sentence because it combines several very common features of Latin grammar.

Because infans would be nominative, and that would suggest the infant is the subject.

But the subject here is aqua.
The infant is not the one being necessary; rather, the water is necessary for the infant.

So Latin uses the dative:

• infanti = for the infant

If you said infans, that would be a different grammatical structure.

Yes, very often.

Adjectives such as necessarius, utilis, idoneus, and some others commonly take the dative to indicate the person or thing affected.

So:

• aqua infanti necessaria est = water is necessary for the infant
• hoc mihi necessarium est = this is necessary for me

This is something English speakers often need to get used to, because English usually uses for instead of a special case ending.

In ordinary prose, it is usually expressed, as it is here: est.

Latin can sometimes omit forms of to be, especially in poetry, sayings, or very compressed style, but for a normal sentence like this, keeping est is the standard and clearest choice.

So Aqua pura infanti necessaria est is the normal full form.

Not always.

Although infans is the source of English infant, in Latin it can refer more broadly to a young child, especially one not yet speaking or very young. Exact age depends on context.

So infanti could be translated as:

• for the infant
• for the baby
• for the child

The best choice depends on the context in which the sentence appears.