The if clause in this verse presupposes that the Jews did not believe Moses. Good News Translation translates it If you had really believed Moses. Jerusalem Bible has “If you really believed him.”

There are difficulties involved in translating a condition contrary to fact in present or past time. The negation of such a condition must sometimes be made explicit in a separate clause, for example, “If you believed Moses—but you really have not believed him—then you would believe me.” It may even be necessary to indicate the negation of the second part of the condition, for example, “you would believe me—but you really do not.” If the second part of the condition is set off by a negative statement, it is necessary to treat the final clause in a somewhat different manner, for example, “It is clear you do not believe Moses, because he wrote about me” or “it is clear you do not believe what Moses wrote, because he wrote about me.” The logical force of this final clause, introduced by because, is made clearer in the following verse.

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 .