Complex sentence: By historical times (c.800-900 AD), all of the West Germanic languages except Old Saxon had dropped the in these words, but not before it had triggered i-Mutation, as well as palatalization in Old English and Old Frisian.
Split into simpler sentences: By historical times (c.800-900 AD), all of the West Germanic languages except Old Saxon had dropped the. The also triggered i-Mutation, as well as palatalization in Old English and Old Frisian.