Seara este plăcută.

Because seara is the definite form of the noun.

• seară = evening / an evening
• seara = the evening

Romanian usually puts the definite article at the end of the noun, not as a separate word like English the.

So:

• o seară = an evening
• seara = the evening

In this sentence, seara is the subject: The evening is pleasant.

Yes. That is a very common source of confusion.

Depending on context, seara can mean:

• the evening as a noun
• in the evening / evenings as a time expression

For example:

• Seara este plăcută. = The evening is pleasant.
  Here, seara is clearly the subject.

• Lucrez seara. = I work in the evening / evenings.
  Here, seara functions more like a time expression.

So in your sentence, it is a noun, not an adverbial expression.

Este is the 3rd person singular present of a fi = to be.

So:

• eu sunt = I am
• tu ești = you are
• el/ea este = he/she is

In Seara este plăcută, este simply means is.

Yes. That is completely normal.

• este = the full form
• e = the short, very common form

So both are correct:

• Seara este plăcută.
• Seara e plăcută.

In everyday speech, e is extremely common.

Because the adjective has to agree with seara, which is feminine singular.

The base adjective is:

• plăcut = pleasant for masculine singular
• plăcută = pleasant for feminine singular

Since seara is feminine singular, you need:

• Seara este plăcută.

Compare:

• Timpul este plăcut. = The weather/time is pleasant.
  (timpul is masculine singular)

So the adjective changes form to match the noun.

A simple pronunciation guide is:

• Seara ≈ SYA-ra or SEA-ra
• este ≈ YES-te
• plăcută ≈ pluh-KOO-tuh

A few useful notes:

• ea in seara is pronounced together, like a glide.
• ă is a very short, neutral vowel, similar to the a in sofa.
• Stress is usually:
  • SEA-ra
  • YES-te
  • plă-CU-tă

Yes. It is the most neutral and natural order.

Romanian often uses:

• subject + verb + adjective/complement

So:

• Seara = subject
• este = verb
• plăcută = predicate adjective

Other word orders are possible for emphasis or style, but this one is the standard, straightforward version.

It can be either, depending on context.

Romanian often uses the definite form for general statements too.

So seara may mean:

• the evening in general, as a part of the day
• this/the evening in a specific context

Without more context, many learners will understand it as a general statement like The evening is pleasant or Evening time is pleasant.

Not normally, if you want a full standard sentence.

Romanian usually keeps the verb to be in this kind of sentence:

• Seara este plăcută.
• Seara e plăcută.

Seara plăcută by itself sounds incomplete as a normal statement. It might appear in special contexts, titles, or fragments, but not as the usual equivalent of The evening is pleasant.

Yes, they do.

Romanian diacritics are part of correct spelling. In this sentence, ă is important:

• plăcută is the correct form

If you write placuta, Romanians will usually still understand you in informal typing, but it is not the proper spelling.

So for correct Romanian, keep the diacritics whenever possible.