Petrecerea începe seara.

Because petrecerea is the definite form: the party.

In Romanian, the definite article is usually attached to the end of the noun, not written as a separate word like English the.

• petrecere = a party / party
• petrecerea = the party

So here, petrecerea is the subject of the sentence: the party.

Începe is the 3rd person singular present tense of the verb a începe, meaning to begin / to start.

Here it agrees with petrecerea, which is singular:

• eu încep = I begin
• tu începi = you begin
• el/ea începe = he/she/it begins
• noi începem = we begin
• voi începeți = you all begin
• ei/ele încep = they begin

So Petrecerea începe = The party begins / starts.

Romanian often uses time expressions like seara without a preposition.

So:

• seara = in the evening
• dimineața = in the morning
• noaptea = at night

This is very natural in Romanian. English needs a preposition, but Romanian often does not.

So Petrecerea începe seara is perfectly normal for The party starts in the evening.

Seara is the definite form of seară.

• seară = evening
• seara = the evening

But in Romanian, the definite form is often used in fixed time expressions, where English translates it more idiomatically:

• seara = in the evening
• not necessarily a literal the evening in English

So although it looks like the evening, in this sentence it functions as a time expression.

Usually, no, not in this general sense.

For a general time-of-day expression, Romanian normally says:

• seara = in the evening

You do use în in more specific phrases, for example:

• în seara asta = this evening
• în seara de luni = on Monday evening

But by itself, în seară is not the normal equivalent of in the evening here.

A simple pronunciation guide is:

pe-tre-CHE-rea în-CHE-pe SEA-ra

A few helpful points:

• ce before e is pronounced like che in check
• î has no exact English equivalent; it is a central vowel sound specific to Romanian
• ea in seara is pronounced together as a glide

So:

• petrecerea sounds roughly like pe-tre-CHE-re-a
• începe sounds roughly like în-CHE-pe
• seara sounds roughly like SEA-ra

Yes, Romanian word order is fairly flexible, but Petrecerea începe seara is the most neutral and natural order here.

It follows the basic pattern:

subject + verb + time expression

You can move parts around for emphasis, but that changes the feel of the sentence. For example, putting seara first would sound more marked, as if you are emphasizing the time.

So for a plain statement, Petrecerea începe seara is the best choice.

By itself, seara usually means in the evening as a general time reference.

If you specifically want this evening / tonight, Romanian more often uses:

• diseară = this evening / tonight

So:

• Petrecerea începe seara = The party starts in the evening
• Petrecerea începe diseară = The party starts this evening / tonight

Context can sometimes make seara more specific, but diseară is the clearer choice for this evening.