Documentul devine public.

Because -ul is the definite article in Romanian.

• document = a document
• documentul = the document

Unlike English, Romanian usually attaches the definite article to the end of the noun instead of putting a separate word before it.

So:

• un document = a document
• documentul = the document

Romanian normally uses a postposed definite article, which means the article comes at the end of the noun.

This is one of the big differences from English:

• English: the document
• Romanian: documentul

This is completely normal in Romanian and happens with many nouns:

• băiatul = the boy
• cartea = the book
• omul = the person / the man

So documentul is not a special case; it follows a very common Romanian pattern.

Devine means becomes.

So the sentence expresses a change of state:

• Documentul devine public. = the document becomes public

If you used este, that would describe a state, not a change:

• Documentul este public. = the document is public

So the difference is:

• devine = enters that state / changes into it
• este = simply is in that state

Because public is an adjective, not a noun here.

In this sentence, public describes the document after the verb devine. It functions as a predicate adjective.

So:

• documentul = noun with definite article
• devine = verb
• public = adjective

You do not use the definite article on the adjective in this structure.

Correct:

• Documentul devine public.

Not correct here:

• Documentul devine publicul.

That last form would look like the public as a noun, which is a different meaning.

Romanian neuter nouns behave like:

• masculine in the singular
• feminine in the plural

Since documentul is singular, any adjective agreeing with it takes the masculine singular form.

So:

• singular: document public
• plural: documente publice

That is why the sentence has:

• Documentul devine public.

not:

• Documentul devine publică.

A deveni means to become.

In devine, you have the 3rd person singular present form, used for:

• he becomes
• she becomes
• it becomes

Here it means it becomes, referring to documentul.

A few present tense forms are:

• eu devin = I become
• tu devii = you become
• el/ea devine = he/she becomes
• noi devenim = we become
• voi deveniți = you become
• ei/ele devin = they become

So devine matches documentul because documentul is singular.

This is the most neutral and natural order:

• subject + verb + adjective

So:

• Documentul = subject
• devine = verb
• public = predicate adjective

Romanian word order is often more flexible than English, but this version is the standard, straightforward one.

Other orders may be possible in special contexts for emphasis, but for a learner, Documentul devine public is the best normal pattern to remember.

It is an adjective.

Here, public describes the state of the document:

• the document becomes public

Romanian often uses adjectives after verbs like:

• a fi = to be
• a deveni = to become
• a părea = to seem
• a rămâne = to remain

So in this sentence, public is not the public as a noun. It is the adjective public.

The plural would be:

• Documentele devin publice.

Here is what changes:

• documentul → documentele = the documents
• devine → devin = become
• public → publice = plural adjective agreement

This is a good example of Romanian agreement:

• singular: Documentul devine public.
• plural: Documentele devin publice.

A simple learner-friendly pronunciation guide would be:

• documentul ≈ do-koo-MEN-too-l
• devine ≈ de-VEE-neh
• public ≈ POO-bleek

A few useful notes:

• Romanian u is usually like oo in food
• Romanian i is usually like ee in see
• Romanian e is usually like e in get or a clear eh
• Final consonants are usually pronounced clearly

A rough full pronunciation:

• do-koo-MEN-too-l de-VEE-neh POO-bleek

Yes. That would mean a document becomes public.

Compare:

• Un document devine public. = A document becomes public.
• Documentul devine public. = The document becomes public.

So the main difference is indefinite vs definite:

• un document = a document
• documentul = the document

Both are grammatically correct; they just mean different things.