It depends on the verb. Some verbs can have both 에게 and 을. And the meaning is different for each case. You must look at the example sentences of each words. - 가르치다 1. (somebody)을 가르치다 means "to educate somebody" 젊은 사람들을 가르치는 것은 매우 의미 있는 일이다 It is a task of great significance to educate young people. 2. (somebody)에게 (something)을 가르치다 means "to teach somebody something" 그녀는 우리에게 과학을 가르쳤다 She taught us science. - 전화하다 is contraction of 전화를 하다 (always used with 을) (somebody)에게 전화하다 means "to give somebody a call" 난 그녀에게 거의 매주 전화했어. I called her almost every week. (Never mind the English word "call her") - 먹이다 1. (someone)에게 (something)을 먹이다 means "to feed something to someone" 여자가 원숭이에게 바나나를 먹이고 있다. She's feeding a banana to a monkey 2. (someone)을 먹이다 means "to support/raise someone" 부인이 그를 먹여 살리고 있다 His wife is supporting him. - 깨우다 (always used with 을) (someone)을 깨우다 means "to wake someone up" 그는 아홉 살짜리 아들을 깨웠다. He woke his nine-year old son.