What is the meaning of "k'Safsechem"?
Radak: The Chaf is a prefix (when); Tafsechem is Mekor (gerund).
What is the meaning of "ki'Dvar Hashem Ta'asu"?
Radak: You will do like you did to Yericho, i.e. burn it.
Why did he say "Re'u Tzivisi Eschem"?
Radak: You should be zealous.
Malbim: Do not say that since you may take spoils, you will take spoils before you burn the city! I commanded you and taught you Hashem's intent (to burn it first).