to teach = عَلَّم
Forms 2 and 4 are both causative in Arabic. So whereas the root verb عَلِم (‘alima) means to know, the Form 2 verb عَلَّم (‘allama) and the Form 4 verb أَعْلَم (‘a’lama) both mean to cause someone to know. However, Form 2 is used for actions which are done repeatedly, and which leave some mark, so عَلَّم (Form 2) is best translated as to teach, as this is a gradual, long-term process, which should leave an indelible mark on the student. أَعْلَم (Form 4) on the other hand does not have this implication, so is best translated as to inform.