Espero que el final no me decepcione.

Because Spanish normally uses esperar que to mean to hope that when a new clause follows.

So:

• Espero = I hope
• que = that
• el final no me decepcione = the ending doesn’t disappoint me / won’t disappoint me

In English, that is often optional: I hope (that) the ending...
In Spanish, que is normally required here.

Because after espero que, Spanish usually uses the subjunctive.

The sentence is expressing a hope about something that is not yet certain, so Spanish treats it as something wished for rather than a plain fact.

• decepciona = present indicative
• decepcione = present subjunctive

So:

• Espero que el final no me decepcione = I hope the ending doesn’t / won’t disappoint me

If you said Espero que el final no me decepciona, it would sound ungrammatical to most speakers.

It is the third-person singular present subjunctive of decepcionar.

The subject is el final, which is singular, so the verb must also be singular.

A quick pattern:

• infinitive: decepcionar
• yo present indicative: decepciono
• present subjunctive stem: decepcion-
• third-person singular present subjunctive: decepcione

So decepcione means may disappoint or, in natural English here, disappoints / will disappoint.

Me means me and is the object pronoun of decepcionar.

The verb decepcionar means to disappoint someone.

So:

• El final me decepciona = The ending disappoints me
• Espero que el final no me decepcione = I hope the ending doesn’t disappoint me

With a full noun, you would often see:

• El final decepciona a Marta = The ending disappoints Marta

With a pronoun, that becomes:

• El final la decepciona = The ending disappoints her

So me is standing in for a mí.

In Spanish, with a normal conjugated verb, the usual order is:

no + object pronoun + verb

So:

• no me decepcione
• no lo sé
• no te entiendo

That is why the negative no comes first, then me, then the verb decepcione.

Here el final means the ending or the finale.

It is a masculine noun, so it takes el:

• el final de la película = the ending of the film
• el final del libro = the ending of the book

So in this sentence, el final most naturally refers to the ending of a story, film, series, game, etc.

Yes, if the context is already clear.

For example, if everyone already knows you are talking about a film or book, you could say:

• Espero que no me decepcione = I hope it doesn’t disappoint me

Spanish often leaves out things that are obvious from context.
In your sentence, el final is included to make it explicit what might disappoint the speaker.

Because the subject of the second action is different.

Compare:

• Espero descansar mañana. = I hope to rest tomorrow.
  Same subject for both verbs: I hope, and I will rest.

• Espero que el final no me decepcione. = I hope the ending won’t disappoint me.
  Different subjects: I hope, but the ending disappoints.

A useful rule:

• same subject -> often infinitive
• different subject -> usually que + subjunctive

Even though espero is present tense, the overall meaning is usually about the future or an outcome that is still unknown.

So Espero que el final no me decepcione usually means:

• I hope the ending doesn’t disappoint me
• I hope the ending won’t disappoint me

Spanish often uses the present subjunctive after verbs of hope to talk about a future possibility.

In standard Spanish, when esperar means to hope, esperar que is normally followed by the subjunctive.

So:

• Espero que venga
• Espero que no llueva
• Espero que el final no me decepcione

If you want the indicative after esperar, that is much less typical and usually involves a different nuance or structure, not this basic I hope that... pattern.

For a learner, the safest rule is:

• espero que + subjunctive

A native speaker would hear it as a very normal way to say:

• I hope the ending doesn’t disappoint me
• I hope the ending won’t let me down

It sounds natural for talking about a book, film, TV series, video game, or anything with an ending you have not reached yet. It implies the speaker is interested but a little worried that the ending may not be satisfying.