And you shall tread down the wicked: You refers to the people faithful to God, and is of course plural. Tread down is a pictorial way of saying “defeat,” and many translators will be able to keep the same figure, as Contemporary English Version does with “trample.” For translators who cannot keep the figure, Good News Translation offers a model with “overcome.”
For they will be ashes under the soles of your feet: This clause continues the picture in verse 1. The wicked will have been burnt up completely, so that they will be like ashes for the faithful to walk over. This interpretation is more relevant to the context than “dust” in Good News Translation. The idea is that the faithful will share in the benefits of the LORD’s victory over the wicked, rather than that they will themselves defeat the wicked. In some languages it will be more natural to translate the metaphor as a simile, “like ashes.” Standing on the enemy is a symbol of victory over them (compare Josh 10.24). In some languages it will be sufficient to translate this clause using “feet” without mentioning soles; for example, “… as though they were ashes under your feet” (Contemporary English Version).
A possible model for the first part of this verse is “Your feet will walk all over the wicked, because they will have been turned to ashes.”
On the day when I act: These words are the same as in 3.17, and here as there are better translated “on the day that I am preparing” (New Jewish Publication Society’s Tanakh; similarly Jerusalem Bible, Bible de Jérusalem, Traduction œcuménique de la Bible).
Says the LORD of hosts: This is the quotation formula that marks the climax and conclusion of the sixth dispute. In many languages it will be best to translate it as a separate sentence, “This is what the LORD Almighty has said” (as in Moffatt, Contemporary English Version, Die Bibel im heutigen Deutsch, Parola Del Signore: La Bibbia in Lingua Corrente).
Quoted with permission from Clark, David J. & Hatton, Howard A. A Handbook on Malachi. (UBS Helps for Translators). New York: UBS, 2002. For this and other handbooks for translators see here .