Breakdown of Það var áreynslan sem gerði afrekið ótrúlegt.
vera
to be
það
it / there (existential) / the one (n)
gera
to do / to make (something) + adjective
sem
who / which / that
ótrúlegur
incredible
áreynslan
the exertion
afrekið
the achievement
Elon.io is an online learning platform
We have an entire course teaching Icelandic grammar and vocabulary.