WordNet sense 1 (cause to feel sure; give reassurance to):

reassure

WordNet sense 2 (inform positively and with certainty and confidence):

tell

WordNet sense 3 (make certain of):

ensure, guarantee, insure, secure

WordNet sense 4 (make a promise or commitment):

promise

WordNet sense 5 (be careful or certain to do something; make certain of something):

ascertain, check, control, see, see to it