εἰ δὲ μή [7 verses](conj particles) This construction  means "otherwise." The three words literally "if however not." The "if" here expresses a condition but it means nothing regarding whether that condition is not when used with the subjective negative. The Greek word "but" means "but," "however," and "on the other hand."  It joins phrases in an adversarial way. Since it always falls in the second position, translating it as "however" often captures its feeling better.  The negative used here is the Greek negative of a subjective opinion, commands, and requests. It is the negative used in "if" phrases. -- The "else" here comes from three Greek words that literally mean "if however not." This is usually translated as "otherwise" or, in the KJV, as "else."

εἰ μὴ [14 verses](conj particle) "Except" is ei me, which is the conjunction that means "if not," and "except." It is also used to create negative questions when the subjective negative would be used in the direct negative question. In negative questions "instead...not" might work better. εἰ is the particle use with the imperative usually to express conditions "if" or indirect questions, "whether." Used with the historical present (present tense in past time) to express unfulfilled conditions and with aorist to refer the past. The mê (me) is the negative used in prohibitions and expressions of doubt meaning "not" and "no."  Followed by an ἀλλὰ clause, the sense is "if not" some word related to time, "at least still," and without the reference time, "at least" but  -  (CW) "But is from Two Greek words are translated as "except." Literally, they mean "if not" but this phrase is used to mean "except." In negative questions "instead...not" might work better. This is not the word usually translated as "but." CW --Confusing Word -- This is not the common word usually translated as "but."

ἰδεῖν [166 verses](verb aor inf act) "To see" is 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." -- The verb translated as "see" means "to see" but it is used like we use the word "see" to mean "to know" or "to perceive." In the past perfect tense, it means "to know," and that sense of knowing gives this word a sense of seeing and understanding. 

εἶδος [1 verse](noun sg neut nom) "Shape" is from eidos, which means "that which is seen", "shape", "form", "beauty of a person", "physique", "kind", "nature," and "wares."

εἰκὼν [3 verses](noun sg fem nom) "Image" is eikôn, which means "likeness," "image," "image in a mirror," "personal description," "semblance," "comparison," and "archetype." It is the source of our word "icon."