Mater dicit ornamenta pretiosa saepe pulchra esse, sed non semper necessaria.

Because Latin usually does not use a separate word like that after verbs of saying, thinking, knowing, and so on.

Instead, it often uses an indirect statement construction:

• a subject in the accusative
• plus an infinitive

So:

Mater dicit ornamenta ... esse

literally works like:

Mother says the ornaments ... to be

but in normal English we translate it as:

Mother says that the ornaments are ...

So the idea of that is present, even though no actual Latin word is written.

In this kind of indirect statement, the subject of the reported idea goes into the accusative, not the nominative.

So in:

• Mater dicit = Mother says
• ornamenta pretiosa saepe pulchra esse = that expensive ornaments are often beautiful

the word ornamenta is the subject of esse, but because it is inside indirect statement after dicit, it appears in the accusative.

A useful thing to know here is that ornamenta could look either nominative plural or accusative plural, because neuter plural forms are often the same in both cases. We know it is accusative from the construction after dicit.

This is a very common point of confusion.

• pretiosa goes directly with ornamenta: expensive ornaments
• pulchra goes with esse: to be beautiful

So the structure is basically:

ornamenta pretiosa = expensive ornaments
pulchra esse = to be beautiful

In other words:

• pretiosa is describing which ornaments we are talking about
• pulchra is part of what the mother says about them

Then the second half works similarly:

• necessaria is also part of what is being said about the ornaments, with esse understood

So:

• expensive ornaments
• are often beautiful
• but not always necessary

Because they all agree with ornamenta, which is neuter plural.

Latin adjectives must agree with the nouns they go with in:

• gender
• number
• case

Here, ornamenta is neuter plural, so the adjectives are also neuter plural:

• pretiosa
• pulchra
• necessaria

For a learner coming from English, this can look strange because English adjectives do not change form. But in Latin, this agreement is normal and very important.

Also, even though pretiosa is more closely attached to the noun and pulchra / necessaria are more predicate-like, all of them still match ornamenta in form.

Because after dicit, Latin normally uses an infinitive in indirect statement, not a finite verb.

So English says:

• Mother says that ornaments are beautiful

But Latin says:

• Mother says ornaments to be beautiful
• that is, ornamenta ... pulchra esse

So esse is used because it is the infinitive to be.
If Latin used sunt, that would be a normal finite verb they are, which is not the usual pattern here.

As for word order: Latin is much freer than English. Putting esse near the end is very common, especially in indirect statement.

Because Latin often leaves out a repeated word when it is easy to understand.

The full idea is:

Mater dicit ornamenta pretiosa saepe pulchra esse, sed non semper necessaria esse.

But repeating the second esse is unnecessary, so Latin omits it:

... sed non semper necessaria.

English does the same sometimes:

• They are often beautiful, but not always necessary

We do not feel a need to repeat are in the second half, because it is understood.

Because non semper means not always.

That is exactly the sense needed here:

• expensive ornaments are often beautiful
• but not always necessary

If you changed the emphasis and said semper non, that would sound more like always not, which is much stronger and not the same idea.

So:

• non semper necessaria = not always necessary
• not never necessary, just not necessary in every case

Latin has no articles like English the, a, or an.

So:

• mater can mean mother, a mother, or the mother
• ornamenta can mean ornaments, the ornaments, or sometimes some ornaments

The context tells you which English wording is most natural.

That is why a Latin sentence can seem a little bare to an English speaker: Latin simply does not require articles.

It can mean either, depending on context.

The Latin present tense often covers both:

• simple present: says
• progressive present: is saying

In a sentence like this, English usually prefers says:

• Mother says that expensive ornaments are often beautiful, but not always necessary.

But in the right context, is saying could also be possible.

So the form dicit itself does not force only one of those two English choices.