Learn Icelandic: Annars vegar hið fyrirsjáanlega, hins vegar hið ófyrirséða. - On the one hand the predictable, on the other hand the unforeseen.

When creating abstract concepts out of adjectives (substantivizing them), Icelandic typically uses the free-standing neuter article hið followed by the adjective, rather than attaching the definite suffix. This structure is the standard way to express concepts like "the predictable" or "that which is unforeseen".

Why do both fyrirsjáanlega and ófyrirséða end in -a here?

Because they follow the free-standing article hið, these adjectives must be in their weak declension. For a neuter singular adjective in the nominative or accusative case, the weak ending is always -a.

Literally speaking, how do annars vegar and hins vegar mean "on the one hand" and "on the other hand"?

The phrase literally translates to "of another way" and "of the other way". The word vegar is the genitive form of vegur (way, road, or direction). In Icelandic, this fixed genitive construction is the standard equivalent of the English phrase.

What is the grammatical difference between the base words fyrirsjáanlegur and ófyrirséður?

Fyrirsjáanlegur uses the adjective suffix -legur to mean "predictable" (able to be foreseen). Ófyrirséður, however, is built from the past participle of the verb að sjá fyrir (to foresee) combined with the negative prefix ó-, literally meaning "unforeseen". Both function as substantivized adjectives in this sentence.