Jhn 3:12 If I have told you earthly things, and ye believe not, how shall ye believe, if I tell you of heavenly things?
Whenever I spoke to you about the earthly, you just didn't trust [in my words]. How then, if I might have spoken to you of the heavenly, might you have trusted [in my words]?"
The Greek word translated as "if" also means "whenever." this is important because Christ is referring in this first phrase to the past. The verbs first sentence are in the aorist tense, which is usually translated as in the past. Here, they clearly refer to what Christ and Nicodemus have been talking about: physical birth and spiritual birth.
Notice that Christ is referring the spiritual birth as something "earthly." Why? Because we get our spiritual life while we are on earth in our bodies.
In the second sentence, Christ changes the Greek word translated as "if" to one that cannot mean "whenever" by using the particle that indicates that something might happen. This is consistent with the verbs used, which are both subjunctive, indicating something that might happen.
The last verse and this both begin with the same Greek word, ei, but they actually are homographs, words spelled the same with different meanings.
εἰ "If" is from ei, which is the particle used to express conditions "if" (implying nothing about its fulfillment) or indirect questions, "whether." It also means "if ever", "in case," and "whenever." It is combined with various conjunctions to create derivative conditions.
εἶπον (1st sg aor ind act) "I have told" is from eipon (eipon), which means "to speak", "to say", "to recite", "to address", "to mention", "to name", "to proclaim", "to plead", "to promise," and "to offer."
καὶ "And" is from kai, which is the conjunction joining phrases and clauses, "and," or "but." After words implying sameness, "as" (the same opinion as you). Used in series, joins positive with negative "Not only...but also." Also used to give emphasis, "even", "also," and "just."
οὐ "Not" is from οὐ ou which is the negative adverb for facts and statements, negating both single words and sentences. The other negative adverb, μή applies to will and thought; οὐ denies, μή rejects; οὐ is absolute, μή relative; οὐ objective, μή subjective.
πιστεύετε (2nd pl pres ind act) "Ye believe" is from pisteuô (pisteuo), which means "to trust, put faith in, or rely on a person", "to believe in someone's words", "to comply", "to feel confident in a thing," and "to entrust in a thing."
"You" is from hymin, which is the 2nd person plural dative pronoun. Dative is the case which indicates to whom something is given.
"Heavenly things" is from epouranios, which is an adjective (used as a noun) means "heavenly", "of the gods" [Homer], and "up to heaven." It means literally "on heaven."
(2nd pl aor subj act ) "Shall ye believe" is from pisteuô (pisteuo), which means "to trust, put faith in, or rely on a person", "to believe in someone's words", "to comply", "to feel confident in a thing," and "to entrust in a thing."
Also, not that the Greek word translated as "believe" has a specific meaning of "trusting in someone's words." Since the verb is used in conjunction with speaking, adding that idea gets us closer to the meaning in Greek.