"We have to be ready in an hour."
Translation:Tenemos que estar listos en una hora.
Had the same question and the best way it was explained to me was sense of obligation. I believe that is what jdrewy is stating. deber: It's my duty to do , or I should do where must is due to obligation but the individual does not 'have to' do it. tener que: I have to do , It is imperative I do ref: http://www.spanishdict.com/answers/240989/deber-v-tener-que