Now (so most translations) is literally “from now” (Goodspeed “from now on”; Phillips “from now onward”). In Matthew 26.29,64, the phrase is used with the meaning “from now on” or “from this time on.” In John’s Gospel it occurs only one other time (14.7). In the present passage the meaning must surely be “now,” as the parallel in 14.29 indicates, where the adverb “now” (Greek nun) is actually used. Some scholars think that the meaning must be “surely,” reaching this conclusion through a comparison with Matthew 26.29,64. (See Matt 23.39 and Rev 14.13 for the only other occurrences of this phrase in the New Testament.)
I tell you this now before it happens may require a slight expansion, for example, “I am telling you now what is going to happen before it happens.”
On the absolute use of the phrase I Am Who I Am, see 4.26. When used without a predicate, this statement is a way of identifying Jesus with God. Both this passage and 8.24 are very close to the Septuagint text of Isaiah 43.10 (“in order that you might know and believe and understand that I am”), where God is the speaker. There are real complications in attempting to translate meaningfully the expression I Am Who I Am. The English rendering certainly does not do justice to the underlying meaning, for in this form it would mean to the average person “I’m just the kind of person I am, and no one else.” While it is grammatically possible in English, in some languages one cannot say I Am Who I Am. In fact in some languages there is no equational verb. Such expressions as “I am John” and “I am good” are simply “I John” and “I good.” Thus a literal rendering of I Am Who I Am would be nothing more than “I who I.” The closest equivalent in some languages is “I am the one who has always existed.” In this way one may emphasize the aspect of “being” and relate it to the divine quality of eternal existence. This means that for the expression “I AM” in the Old Testament, one may translate “the eternally existing one.”
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 .