Din moment ce funcționarul a autentificat copia legalizată, dosarul este complet.

Here din moment ce is a fixed conjunction meaning since, given that, or seeing that.

In this sentence, it introduces a reason:

• Din moment ce funcționarul a autentificat copia legalizată... = Since the official has authenticated the legalized copy...

It is not being used literally as from the moment that here. In other contexts, din momentul în care is the clearer expression for a literal time meaning.

The comma separates the subordinate clause from the main clause:

• Din moment ce funcționarul a autentificat copia legalizată,
• dosarul este complet.

Romanian normally uses a comma when a subordinate clause comes before the main clause, as it does here.

Because Romanian usually puts the definite article at the end of the noun, not before it.

• funcționar = official / clerk
• funcționarul = the official / the clerk

So -ul is the masculine singular definite article.

The same thing happens in:

• dosar → dosarul = the file / the dossier
• copie → copia = the copy

A autentificat is the perfectul compus form, the most common Romanian past tense in everyday use.

It is made of:

• the auxiliary a = has
• the past participle autentificat = authenticated

So:

• a autentificat = has authenticated / authenticated

This tense is used for a completed action in the past. In this sentence, that completed action explains the present result:

• the authentication happened,
• and now the file is complete.

Because copia legalizată is an inanimate direct object.

In Romanian, pe is often used before direct objects that are people, pronouns, or certain highly specific/marked objects. But with ordinary inanimate nouns, it is usually not used.

So:

• funcționarul a autentificat copia legalizată = correct

Using pe here would sound unnatural in standard Romanian.

Because copia legalizată is a noun plus an adjective, and the adjective must agree with the noun.

• copia is feminine singular definite
• legalizată is also feminine singular

So they match.

Breakdown:

• copie = copy
• copia = the copy
• legalizat = legalized, certified
• legalizată = feminine singular form of that adjective/participle

If the noun changed, the adjective would change too:

• documentul legalizat = the legalized document
• copiile legalizate = the legalized copies

They are related, but not identical.

• a autentifica = to authenticate, verify officially
• a legaliza = to legalize, certify in a legally valid way, often in bureaucratic or notarial contexts

So copia legalizată is a certified/legalized copy, and a autentificat refers to the official act of authentication.

In administrative language, Romanian often uses several precise legal verbs that may sound a bit overlapping in English. So the combination may sound bureaucratic, but it is natural in formal/legal contexts.

Because complet here is an adjective used after the verb este.

After a fi = to be, Romanian uses the plain adjective form:

• dosarul este complet = the file is complete

You do not add the definite article to the adjective there.

Compare:

• dosarul complet = the complete file
  
  • adjective directly describing the noun
• dosarul este complet = the file is complete
  
  • adjective after to be

Because they mean different things.

• complet = complete
• completat = filled in / filled out / completed

So:

• dosarul este complet = the file contains everything necessary
• formularul este completat = the form has been filled out

In your sentence, the idea is that the dossier is complete, not that it has been filled in.

Because it agrees with dosarul, which is masculine singular.

So:

• dosarul este complet = masculine singular

If the subject were feminine, the adjective would change:

• cererea este completă = the application is complete

If plural:

• dosarele sunt complete = the files are complete

Yes. Romanian word order is fairly flexible.

You can also say:

• Dosarul este complet, din moment ce funcționarul a autentificat copia legalizată.

This still makes sense, but the emphasis changes a little.

• Starting with Din moment ce... highlights the reason first
• Starting with Dosarul este complet... highlights the result first

The original version sounds natural and slightly more formal/explanatory.

A few useful pronunciation points:

• ț = ts, like the end of cats
  • funcționarul
• ă = a short neutral vowel, similar to the a in sofa
  • legalizată
• ce is pronounced like che in check, without the final ck
  • ce

Very rough pronunciation guides:

• funcționarul ≈ foonk-tsyo-NA-roo l
• legalizată ≈ le-ga-li-ZA-tuh
• dosarul ≈ do-SA-rul
• autentificat ≈ au-ten-ti-fi-CAT

These are only approximations, but they help an English speaker get close.