Learn Arabic (Modern Standard): أعجبني الفيلم كثيرا.

In Arabic, the concept of "liking" something is usually expressed by saying that the thing pleased you. Here, the verb is أعجب (pleased), the attached pronoun ني means "me", and الفيلم (the film) is the subject doing the pleasing. Literally, the sentence reads: "Pleased me the film."

Why is the suffix for "me" ني (-ni) instead of just ي (-i) like in كتابي (my book)?

When you attach "me" to a noun to mean "my", you use ي (-i). But when you attach it to a verb as the receiving object (like "pleased me"), Arabic adds a letter nun before it to protect the verb's final vowel from changing. This makes the suffix ني (-ni) on verbs.

Why does كثيرا end with an extra alif?

The base word is كثير (much). When used as an adverb to describe how an action happened (meaning "a lot" or "very much"), it takes the accusative case. In Arabic writing, indefinite accusative words get an "-an" sound at the end, which is usually written with two fat-has on top of an added alif: كثيرا.

If I wanted to say "I liked the story" instead, would the verb أعجبني stay the same?