οἶδεν [38 verses](3rd sg perf ind act) "Know" is oida which has the sense of "to know." This listing is not a root word, but the past perfect tense of eido, which means "to see," "to examine," "to perceive," "to behold," "to know how to do," "to see with the mind's eye," and "to know." That which "has been seen" is that which is "known." This is a somewhat legalistic idea because the truth can only be established by eyewitnesses. Since the past perfect of "see" forms the present tense of "know" the pluperfect tense used here, which indicates an action completed before some other action in the past, is used as the simple past tense, "knew." -- The word translated as "know" means primarily "to see" and is used to mean "know' as we use the word "see" to mean "know" in English. It is the past perfect tense so "have seen" but it is translated as the present tense of "know."  What someone "has seen" is what they "know" in the present.