You will look for me has reference to the time after Jesus has returned to the Father. The implication is that there will be a time when they will look for Jesus (to help them) but it will be too late. It is, of course, in contrast to the present when they are looking for him in order to kill him.

Where I will be is literally “and where I am.” Again the present tense “I am” has a future reference and is better translated I will be. It is possible, though not probable, that “I am” may really be “I go.” In Greek these two verbs are spelled alike except for the accent, but very few commentators and apparently no translators understand the verb here to mean “I go.” The point is that Jesus will be with the Father after his death, but these people cannot be with the Father even if they die. In the clause you cannot go where I will be, the pronouns I and you are emphatic, in strong contrast to each other.

Quoted with permission from Newman, Barclay M. and Nida, Eugene A. A Handbook on the Gospel of John. (UBS Handbook Series). New York: UBS, 1980. For this and other handbooks for translators see here .