Acesta este secretul.

• Acesta = this / this one
• este = is
• secretul = the secret

So the sentence is literally This is the secret.

Because Romanian usually puts the definite article at the end of the noun.

• secret = a secret / secret
• secretul = the secret

That -ul is the definite article here. This is very common in Romanian:

• băiat = boy
• băiatul = the boy

So secretul means the secret, not just secret.

Because acesta is a demonstrative pronoun: it means this one / this and stands by itself.

Compare:

• acest secret = this secret
  Here acest is a demonstrative adjective and must go with a noun.

• Acesta este secretul. = This is the secret.
  Here acesta stands alone, so it is a pronoun.

So:

• acest
  • noun
• acesta = stands alone

Secret is actually a neuter noun in Romanian.

Romanian neuter nouns behave:

• like masculine in the singular
• like feminine in the plural

So in the singular, a neuter noun takes masculine-looking forms:

• un secret
• secretul
• acesta

That is why Acesta este secretul uses acesta, not a feminine form.

A simple learner-friendly pronunciation is:

a-CHES-ta YES-te se-CRE-tul

A few helpful notes:

• ce in acesta sounds like che in chess
• este is often pronounced roughly YES-te
• stress falls roughly on:
  • aCHESta
  • YESte
  • seCREtul

Yes. That is a very common, natural, more conversational version.

Compare:

• Acesta este secretul. = more careful, fuller, slightly more formal
• Asta e secretul. = very common in everyday speech

Also:

• este is the full form
• e is the shorter spoken form

So both are correct, but they feel slightly different in style.

Yes, Romanian word order is somewhat flexible.

• Acesta este secretul. = This is the secret.
• Secretul este acesta. = The secret is this.

Both are grammatical, but the emphasis changes.

The original sentence puts focus on acesta as the thing being identified.
The reversed version puts focus on secretul first, then identifies it.

Because Romanian usually attaches the to the noun instead of using a separate word like English does.

English:

• the secret

Romanian:

• secretul

So Romanian does not need a separate word before the noun here.

Because acesta is already a demonstrative word, and demonstratives are inherently definite.

In English, this is already definite too:

• this secret
• not normally the this secret

Romanian works similarly.
Acesta already means this one / this, so it does not need another definite marker.

The most basic form is:

• acest secret = this secret

That is different from the original sentence:

• Acesta este secretul. = This is the secret.

So compare:

• acest secret = noun phrase
• acesta = pronoun

Romanian also often allows a postposed form in everyday speech:

• secretul acesta = this secret

So you may see both:

• acest secret
• secretul acesta

In a sentence like this, este is normally used.

• Acesta este secretul.
• Asta e secretul.

Romanian usually does not drop the verb to be in standard sentences of this kind. So unlike some languages, you normally need este / e here.