For if ye turn away
shuwb  (shoob)
to turn back (hence, away) transitively or intransitively, literally or figuratively
from after
'achar  (akh-ar')
the hind part; generally used as an adverb or conjunction, after (in various senses)
him he will yet again
yacaph  (yaw-saf')
to add or augment (often adverbial, to continue to do a thing)
yanach  (yaw-nakh')
to deposit; by implication, to allow to stay
them in the wilderness
midbar  (mid-bawr')
a pasture (i.e. open field, whither cattle are driven); by implication, a desert; also speech (including its organs) -- desert, south, speech, wilderness.
and ye shall destroy
shachath  (shaw-khath')
to decay, i.e. (causatively) ruin -- batter, cast off, corrupt(-er, thing), destroy(-er, -uction), lose, mar, perish, spill, spoiler, utterly, waste(-r).
all this people
`am  (am)
a people (as a congregated unit); specifically, a tribe (as those of Israel); hence (collectively) troops or attendants; figuratively, a flock -- folk, men, nation, people.