Learn Farsi: قبلا این فیلم را دیدم.

قبل از means before a specific event and always needs a noun or pronoun right after it. Here, before just means previously in a general sense. For this stand-alone meaning, you use the adverb قبلا.

What is the pronunciation of the ending on قبلا?

It is pronounced with an -an sound, making the whole word ghablan. This ending is a special marker borrowed from Arabic. It turns the base word قبل into an adverb meaning previously.

Why do we need the word را in this sentence?

را is the specific direct object marker. Because you are talking about seeing a specific movie (این فیلم), Farsi grammar requires you to tag it with را so we know exactly what was seen.

Why isn't the word for I (من) at the beginning of the sentence?

In Farsi, you do not need to use the subject pronoun if it is already clear. The verb دیدم ends in -م, which tells us that I did the seeing. Adding من at the start is optional and usually only done for extra emphasis.