To return is regresar. I have heard that when there is a stress on e, it caves in and it turns into ie. For instance querer become quiero because at present tense the penultimate syllable is stressed and it should have been quero but the e part caves in.
Why doesn't this happen in the case of regresar. You would think it should have become regrieso but it is in fact regreso. Can anyone explain why?