Learn Swedish: Det här är den allra viktigaste frågan. - This is the very most important question.

Allra is an adverb used to make a superlative even stronger. While den viktigaste frågan means 'the most important question', adding allra makes it den allra viktigaste frågan—'the very most important question' or 'the most important question of all'. It always goes directly before the superlative adjective.

Why is the adjective spelled viktigaste instead of viktigast?

Because it is part of a definite noun phrase. When a superlative is used directly before a noun with the definite article (den), it must take the definite ending -e. You would only use viktigast if it were standing alone after a verb, like in Frågan är viktigast (The question is most important).

Why does the sentence use det här when frågan is an en-word?

When you use 'this' as a dummy pronoun to introduce something (like 'This is...'), Swedish always uses the neuter form det här, no matter the gender of the noun that comes later in the sentence. You would only use den här if it were attached directly to the noun, like in Den här frågan är viktig (This question is important).

Why do we need both den and the -an ending on frågan?