IE people meet Latin and parchment

Latin was a framework for inserting stem words.  Local IE, living to the north where papyrus never lasted suddenly had a written frame work, adaptable. If you run, and Latin lets you 'er' it, then become a runner. The core stems for the IE people remain, they can add the tense suffixes, which likely were similar in any event.

Latin was a set of dictionary rules, optimized for parchment. Fit your stem into the rule set.