Мне понравилась эта пьеса, потому что финал был неожиданным.

Because понравиться / нравиться works differently from English to like.

In Russian, the thing that is liked is the grammatical subject, and the person who experiences the feeling is put in the dative case.

So:

• Мне понравилась эта пьеса = literally something like To me, this play was pleasing
• мне = to me
• эта пьеса = the thing that pleased me

That is why you use мне, not я.

Compare:

• Мне нравится музыка = I like music
• Ему понравился фильм = He liked the film

Because the verb agrees with the subject, and the subject here is эта пьеса.

• пьеса is a feminine singular noun
• therefore the past-tense verb must also be feminine singular

So:

• понравился → masculine
• понравилась → feminine
• понравилось → neuter

Since пьеса is feminine, the correct form is понравилась.

This is an aspect difference.

• нравиться = imperfective
• понравиться = perfective

In this sentence, понравилась means that the speaker had a reaction to the play as a completed event: I liked the play / the play appealed to me.

Very roughly:

• Мне нравится эта пьеса = I like this play / I am fond of this play
• Мне понравилась эта пьеса = I liked this play

The perfective verb is very natural when talking about your impression after seeing or reading something.

Because it is the subject of the verb понравилась.

Even though English says I liked the play, Russian structures it differently:

• мне = experiencer, in the dative
• эта пьеса = subject, in the nominative

So эта пьеса stays in nominative because it is the thing that pleased the speaker.

Also notice agreement:

• эта is feminine singular nominative
• пьеса is feminine singular nominative

They match.

эта means this and agrees with пьеса.

Since пьеса is:

• feminine
• singular
• nominative

the demonstrative must also be:

• feminine
• singular
• nominative

That is why it is эта.

Compare:

• этот фильм = this film
• эта пьеса = this play
• это стихотворение = this poem
• эти книги = these books

Because потому что introduces a subordinate clause meaning because.

The sentence has two parts:

1. Мне понравилась эта пьеса
2. потому что финал был неожиданным

Russian normally separates this kind of because-clause with a comma.

So the comma here is standard punctuation.

Because after быть in the past or future, Russian often uses the instrumental case for the predicate complement.

Here:

• финал = subject, nominative
• был = was
• неожиданным = unexpected, in instrumental

So был неожиданным means was unexpected.

This pattern is very common:

• Он был врачом = He was a doctor
• Она была счастливой = She was happy
• Финал был неожиданным = The ending was unexpected

With adjectives, Russian can also sometimes use a short form:

• финал был неожиданен

But был неожиданным is very natural and common.

Because финал is a masculine singular noun.

In the past tense, Russian verbs agree in gender and number with the subject:

• был = masculine singular
• была = feminine singular
• было = neuter singular
• были = plural

Since финал is masculine, the correct form is был.

Yes, but it would mean something slightly different.

• Мне понравилась эта пьеса = I liked this play
• Мне понравился этот финал = I liked this ending

In the original sentence, the whole play is what pleased the speaker, and the reason is that the ending was unexpected.

So the structure is:

• I liked the play
• because the ending was unexpected

If you say Мне понравился этот финал, you are making the ending itself the main thing you liked.

Yes. Russian word order is fairly flexible, although the basic version here is completely natural.

Original:

• Мне понравилась эта пьеса, потому что финал был неожиданным.

Possible variations:

• Эта пьеса мне понравилась, потому что финал был неожиданным.
• Потому что финал был неожиданным, мне понравилась эта пьеса.

The meaning stays similar, but the emphasis changes.

For a learner, the original order is the safest and most neutral.

Yes, usually пьеса means a play in the theatrical/literary sense.

So this sentence is about a dramatic work, not a movie or a musical recording.

For comparison:

• пьеса = play
• фильм = film/movie
• спектакль = stage performance/production
• роман = novel

So if you are talking about the written or dramatic work itself, пьеса is the right word.