È stata un'esperienza orribile.

È stata is the passato prossimo of essere and means was / has been in the sense of a completed past event.

• È stata un'esperienza orribile. = It was a horrible experience.
• It suggests the experience is finished.

Era is the imperfetto, which is often used for:

• background description,
• ongoing past situations,
• repeated past states.

So:

• È stata un'esperienza orribile. = a finished event, viewed as a whole.
• Era un'esperienza orribile. = more like describing something in the past as being horrible, often as background or an ongoing situation.

Stata is the past participle of essere used with a feminine singular subject or complement.

The verb phrase is:

• è = is/has (3rd person singular of essere as auxiliary)
• stata = been

Together, è stata literally corresponds to has been, though in English the natural translation here is usually was.

Because it agrees with un'esperienza, which is a feminine singular noun.

In Italian, with essere, the past participle normally agrees in gender and number with the subject.

Here the subject is essentially un'esperienza, so you get:

• masculine singular: stato
• feminine singular: stata
• masculine plural: stati
• feminine plural: state

Since esperienza is feminine singular, the correct form is stata.

This is because of elision.

Una becomes un' before a word that begins with a vowel:

• una casa
• un'amica
• un'esperienza

So un'esperienza is just the normal written form of una esperienza with the vowel dropped for smoother pronunciation.

The apostrophe is important here.

No. They look similar, but they are not the same thing.

• un without an apostrophe = usually masculine singular
  • un libro
  • un uomo
• un' with an apostrophe = feminine singular, from una
  • un'amica
  • un'esperienza

So in this sentence, un'esperienza is feminine, not masculine.

The accent distinguishes the verb è (is) from e (and).

• è = is
• e = and

So:

• È stata... = It was...
• E stata... would be incorrect.

In standard Italian writing, that accent should be included.

Italian often omits subject pronouns when they are not needed, because the verb form usually gives enough information.

In English you must say it was.
In Italian, you can simply say:

• È stata un'esperienza orribile.

There is no separate word for it here. The sentence is understood naturally without it.

Yes, it can go before or after, but after the noun is the most neutral and common position here.

• un'esperienza orribile = neutral, straightforward
• un'orribile esperienza = also correct, but can sound a bit more emphatic or literary

In Italian, adjective position can affect style and nuance. For a simple statement, noun + adjective is very common.

A rough pronunciation guide:

• È ≈ eh
• stata ≈ STAH-tah
• un'esperienza ≈ oo-ne-speh-RYEN-tsa
• orribile ≈ or-REE-bee-leh

A few notes:

• rr in orribile is rolled or tapped, depending on accent.
• z in esperienza is pronounced like ts.
• Stress falls on:
  • sta in stata
  • rien in esperienza
  • ri in orribile

It is neutral standard Italian. It works in both spoken and written language.

You could say it:

• in conversation,
• in writing,
• in a review,
• when describing a trip, movie, job, meal, etc.

It does not sound especially formal or especially casual.

Yes. Orribile and terribile are both common and both can mean horrible/terrible.

Very roughly:

• orribile often feels strongly negative, like horrible
• terribile can mean terrible, and depending on context it can sometimes be a bit broader or slightly less graphic

In many everyday contexts, they are interchangeable:

• È stata un'esperienza orribile.
• È stata un'esperienza terribile.

Both are correct.

Yes. Esperienza is a broad word. It can refer to many kinds of experiences, for example:

• a trip,
• a meal,
• a conversation,
• a job interview,
• a medical situation,
• a class,
• a customer-service interaction.

So the sentence can be used in many everyday situations, not only for something extreme.

Yes, Italians really say this. It sounds natural.

It is a normal, idiomatic way to comment on something negative in the past. Depending on context, a speaker might also say:

• È stata davvero un'esperienza orribile. = It was a really horrible experience.
• Che esperienza orribile! = What a horrible experience!
• È stata un'esperienza pessima. = It was an awful experience.

But your sentence itself is completely natural.