As you may know, there are no tenses in Hebrew, but rather only aspects.
The perfect aspect is frequently translated as a past tense into English.
The imperfect aspect is frequently translated as a future tense. The imperfect
aspect often carries an imperative sense in the Old Testament. For instance,
the imperfect is used in the Ten Commandments. The translators of the
Hebrew Bible into Greek often used the future tense for the Hebrew imperfect.
This is the case with the LXX. Any author writing in Greek who was influenced
by the LXX, as were most, if not all of the authors of early Christian
writings would have taken characteristics of Hebrew grammar over into their
own writings. This is, probably, what has happened in the case you cite
from John 14:15.
David B. Peabody
Nebraska Wesleyan University