Learn Hardcore Russian: Заявитель предъявил оригинал документа. - The applicant presented the original document.

заявитель is nominative singular because it is the subject.
оригинал is accusative singular because it is the direct object of предъявил.
документа is genitive singular because Russian says оригинал чего? = the original of what?

So the structure is basically:

[subject] + [verb] + [direct object] + [genitive complement]

Why does оригинал look the same in the accusative as in the nominative?

Because оригинал is a masculine inanimate noun.

In Russian, for masculine inanimate nouns in the singular, the accusative form is the same as the nominative form.

So:

If it were animate, the accusative would usually match the genitive instead.

Why is it документа, not документ?

Because after оригинал Russian normally uses the genitive:

So документа answers the question чего? (of what?).

This is very common in official and formal Russian.