But abide thou (su de mene). Emphatic contrast (su de), “But thou.” Present active imperative of menō, common verb, to remain.
In the things which (en hois). The antecedent to hois is not expressed (“in which things”) and the relative is attracted from ha accusative with emathes (didst learn, second aorist active indicative of manthanō) to the case of the unexpressed antecedent (locative with en).
Hast been assured of (epistōthēs). First aorist passive indicative of pistoō, old verb (from pistos, faithful), to make reliable, only here in N.T.
Knowing from whom (eidōs para tinōn). Second perfect active participle of oida. Note tinōn (ablative case after para in an indirect question). The list included the O.T. prophets, Paul, Eunice, Lois. There ought to be moral authority in such personages.