adverse, averse
adverse means unfavorable (she had an adverse reaction to the drug)
averse means opposed to (he was averse to going into deep water)

anxious, eager
anxious means worried (he was anxious about the surgery)
eager means enthusiastic desire (she was eager to start college)

assure, ensure, insure
assure means to put at ease (he was able to assure them of success)
ensure means to guarantee (the lab results could ensure the purity of the milk)
insure means cover with insurance (they can insure your loss)