Contd.
The use of these two (and several others, such as nakah and chalal) words depends, in large part, on the context. Keep in mind that Hebrew (Paleo or Modern) has no vowels and only twenty-two letters in its alphabet, therefore context plays a great deal of importance as does vocalizations.
The context of a prohibition of doing what we would describe as murder, as used in Exodus, Deuteronomy and Yeshua's quote of the Torah in Matthew (New Covenant, which translates from the Greek φονεύω or phoneuō, also translates, properly, to English as our concept of murder.
To form the imperative in Hebrew, only (in most cases) a word or a phrase ending or beginning syllable is modified, although, (again, in most cases) the word or root-word itself is not changed. The imperative is almost always associated with the future tense in use. Thus, the change from "You shall not do murder" is modified by the dropping of a single letter (in this case the Hebrew equivalent of a) to become nearly our rendition of "You cannot do murder").
I apologize for the lack of clarity, but this subject usually takes a large part of several Chapters in learning the Hebrew language...