Cretio forma aditionis hereditatis est.

It is a simple copular sentence: X est Y = X is Y.

• Cretio = one noun in the nominative
• forma = another noun in the nominative
• est = is
• aditionis hereditatis = a genitive phrase that goes with forma

So the sentence has the pattern Cretio is a form of acceptance of inheritance.

Here is a word-by-word parsing:

• Cretio — nominative singular, feminine; from cretio, cretionis
• forma — nominative singular, feminine; from forma, formae
• aditionis — genitive singular, feminine; from aditio, aditionis
• hereditatis — genitive singular, feminine; from hereditas, hereditatis
• est — 3rd person singular present indicative of sum, esse, meaning is

So grammatically, the backbone is:

• Cretio ... est forma
• with aditionis hereditatis attached to forma

Because after sum (to be), Latin normally uses the nominative for both the subject and the predicate noun.

This is the same idea as in:

• Cicero orator est = Cicero is an orator
• Roma urbs est = Rome is a city

So here:

• Cretio = the thing being talked about
• forma = what it is said to be

This second nominative is often called a predicate nominative.

Strictly speaking, with sum, Latin can leave that a little flexible, because both nouns are nominative.

But in this sentence, context makes cretio the most natural subject, because the sentence is clearly defining that term:

• Cretio = the term being explained
• forma aditionis hereditatis = the category or definition

So the natural understanding is:

• Cretio is a form of acceptance of inheritance

rather than

• A form of acceptance of inheritance is cretio

That second version is not impossible in logic, but it is not the natural way to read this sentence in context.

They form a nested genitive construction.

First:

• aditio hereditatis = acceptance of inheritance

Here hereditatis depends on aditionis.

Then that whole phrase depends on forma:

• forma aditionis hereditatis = a form of acceptance of inheritance

So the structure is:

• forma
  
  • aditionis
    • hereditatis

In other words, hereditatis does not directly describe forma first; it most naturally goes with aditionis.

The genitive often corresponds to English of.

In this sentence:

• aditionis = of acceptance / of entering upon
• hereditatis = of an inheritance

So Latin builds up the meaning through of-relationships:

• forma aditionis hereditatis
• literally, a form of acceptance of inheritance

English may smooth that out a little, but the Latin grammar is straightforwardly genitive.

Yes. Latin word order is much more flexible than English word order.

Placing est at the end is very normal, especially in formal or written Latin. The sentence could also appear as:

• Cretio est forma aditionis hereditatis

and the meaning would stay the same.

Latin uses endings, not just word order, to show grammatical roles. So est can move without causing confusion.

Because Latin has no articles.

Latin does not have separate words for a, an, or the. Whether you translate forma as:

• a form
• the form

depends on context.

In a sentence like this, where a term is being defined, English usually prefers a form, so that is the most natural translation.

No, it is a technical legal term.

That matters because the sentence is not casual everyday Latin; it sounds like the language of law or legal explanation. A learner may find it slightly unusual for that reason.

So the sentence is functioning almost like a definition:

• cretio = a particular legal form or method connected with accepting an inheritance

That helps explain why the wording is compact and abstract.