Should or Have to?

It was brought to my attention recently that a large number of Japanese English Language Learners often confuse the modals, should and have to. Having worked with many Japanese students for years now, I was initially skeptical about this generality since I had never noticed it before and always thought of myself as a keen observer of tendencies particular to certain groups of Learners. The individual who brought this point up is herself Japanese and admitted to often being confused as to how to use should and have to, properly. So we decided to conduct a quick unscientific survey and sequestered three other Japanese English Learners and asked them "what is the difference between should vs. have to?". And as Mama predicted, all three of them thought that should was used for certainty/necessity/obligation while have to was used for advice/opinion. I don't want to suggest that my 5 minutes of research should be taken as Law but maybe there is something to it. So, if a Japanese ESL student says to you: "I have to phone home since I should work overtime" don't misunderstand, your boss is not living with them. Now you know.