Learn Dutch: Het voorval is volledig onverklaarbaar. - The incident is completely inexplicable.

No, it remains volledig. When volledig (completely) is used as an adverb of degree modifying an adjective (onverklaarbaar), it never gets an -e ending. You would write het volledig onverklaarbare voorval. Only the main adjective describing the noun gets the -e.

What is the difference between het voorval and de gebeurtenis?

Both translate to 'event' or 'incident'. However, het voorval specifically refers to a sudden, often unexpected or unusual incident (like a strange occurrence or a minor accident). De gebeurtenis is a broader, more neutral word for any event, whether planned or unplanned.

How do you break down the word onverklaarbaar?

It is built around the verb verklaren (to explain). Adding the suffix -baar (-able) creates verklaarbaar (explainable). Adding the negative prefix on- (un- or in-) makes it onverklaarbaar (inexplicable).

Why doesn't onverklaarbaar get an -e ending here, since voorval is a het-word?

Adjectives only get an -e ending when they are placed directly before the noun they describe (for example, het onverklaarbare voorval). When the adjective comes later in the sentence after a verb like is, it never gets an -e ending, regardless of the noun's gender.