No me gusta ver una película de terror cuando estoy solo.

In Spanish, gustar doesn’t work like “to like” in English.

• Literally, me gusta means “it pleases me”, not “I like it”.
• So the thing that is liked is the grammatical subject, and the person who likes it is an indirect object.

In this sentence:

• ver una película de terror = the thing that “pleases”
• me = “to me”
• gusta = “is pleasing”

So no me gusta = “it is not pleasing to me” → “I don’t like it.”
Saying no gusto would mean “I am not pleasing” (to someone), which is not what you want here.

Me is an indirect object pronoun meaning “to me”.

With gustar, you usually say:

• Me gusta = It is pleasing to me → I like it.
• No me gusta = It is not pleasing to me → I don’t like it.
• Te gusta = It is pleasing to you.
• Le gusta = It is pleasing to him / her / you (formal).

So me answers “to whom is it pleasing?”

The form of gustar agrees with the thing that is liked (the subject), not the person who likes it.

Here, the subject is the whole action:

• ver una película de terror = “watching a horror movie” (an activity)

That is treated as singular, so you use gusta (3rd person singular).

Compare:

• Me gusta la película. – I like the movie. (singular thing → gusta)
• Me gustan las películas. – I like the movies. (plural things → gustan)
• Me gusta ver películas de terror. – I like watching horror movies. (watching = one activity → gusta)

After gustar (and many other verbs), when you talk about liking an activity, you use the infinitive:

• Me gusta ver… – I like to watch…
• Me gusta leer… – I like to read…
• Me gusta cocinar… – I like to cook…

If you said no me gusta veo, it would be ungrammatical.
You can either:

• Use gustar + infinitive:
  No me gusta ver películas de terror.
• Or use a different structure:
  No veo películas de terror cuando estoy solo. – I don’t watch horror movies when I’m alone.

Both can involve using your eyes, but:

• Ver = to see / to watch (more general, often just perceiving with the eyes; also the standard verb for “to watch TV / movies”).
• Mirar = to look (at), to watch (more active, focusing attention on something).

In the context of movies, ver una película is the more neutral and common option, especially in Latin America.
You can say mirar una película, and it’s understood, but ver una película is more standard.

• Una película de terror = a horror movie (any horror movie, not specific).
• La película de terror = the horror movie (a specific one that both speakers know about).

In the original sentence, we’re talking about horror movies in general when alone, so una (indefinite) is more natural. If you meant a particular movie, you could say:

• No me gusta ver la película de terror cuando estoy solo.
  (e.g., a specific scary movie you both know).

De terror literally means “of terror”, but functionally it means “horror (genre)”.

So:

• una película de terror = a horror movie

You could also see:

• una película de miedo – literally “a movie of fear,” also used for horror movies.
• una película terrorífica – “a terrifying movie” (focus on how scary it is, not necessarily the genre label).

For genre, película de terror is the most standard phrase.

Spanish uses estar (not ser) for temporary conditions or states, including:

• physical location,
• emotional/mental states,
• being alone (as a temporary situation).

So:

• Estoy solo. – I am alone (right now / in this situation).
• Soy solo. – Sounds strange; would suggest a more essential trait, like “I am a lone person by nature,” and even then it’s not a typical way to say it.

For “when I’m alone (at that moment),” you must use estar → cuando estoy solo.

Solo / sola must agree in gender with the person it describes.

• If the speaker is male: cuando estoy solo.
• If the speaker is female: cuando estoy sola.

The sentence as written assumes a male speaker. A female speaker should say:

• No me gusta ver una película de terror cuando estoy sola.

Yes, and it’s very natural:

• No me gusta ver películas de terror cuando estoy solo.
  = I don’t like watching horror movies when I’m alone.

Both are correct:

• una película de terror – a horror movie (singular, generic).
• películas de terror – horror movies (plural, generic).

The meaning is practically the same in this context; plural often sounds more general.

Yes, that’s fine:

• Cuando estoy solo, no me gusta ver una película de terror.

Spanish allows quite flexible word order for adverbial phrases like cuando estoy solo. Putting it first can slightly emphasize the condition (“When I’m alone, I don’t like…”), but the meaning doesn’t change.

You use the indicative (here: estoy) with cuando when you’re talking about:

• habits,
• general truths, or
• real, completed events.

In this sentence, you’re describing a habitual situation:

• When I’m alone (in general), I don’t like watching horror movies.

So cuando estoy solo (indicative) is correct.

You’d use the subjunctive after cuando if you were talking about a future, not-yet-real action in certain constructions:

• Cuando esté solo, no voy a ver una película de terror.
  When I’m (later) alone, I’m not going to watch a horror movie.

No. In Spanish, you need “no” to make the verb negative. The me is not negative; it just means “to me.”

• Me gusta ver… – I like to watch…
• No me gusta ver… – I don’t like to watch…

A real double negative would be something like:

• No me gusta nada. – I don’t like anything.
  (literally: “Nothing pleases me.”)

But that’s normal and correct in Spanish. Your sentence has just one negation (no), which is required.