intellego

Intellego, intellegere, intellexi, intellectum is third conjugation, inter- plus lego, "gather": to understand is to pick out one thing from among others. The perfect intellexi shows the same x for g-plus-s that you get in rexi from rego.

Spelling first, because the corpus is full of both: intellego is the classical form and intelligo the later and medieval one, and you will see intelligere, intelligitur, intelligentia far more often than the -e- forms in anything written after antiquity. They are one word. It takes an accusative and infinitive for reported thought (intellexit rem esse gravem) or an indirect question with the subjunctive. English intellect, intelligent and intelligence all descend from it, including the spy sense, which is the "gathering" root showing through again.