* You can say 먹다 not only for meals but for snacks as well. But 식사하다 means to have meals. So you cannot use 식사하다 for having snacks. The other difference is 먹다 is a native Korean word and 식사 is a Sino-Korean word which means it's from China. It's more likely used in formal setting compare to native Korean words. e.g. 나는 지금 간식 먹고 있어. (I'm having my snack now.) e.g. 저녁 먹었어? ( Did you have dinner?) -casual e.g. 저녁 먹었어요? (Did you have dinner?) -polite e.g. 저녁 드셨어요? (Did you eat dinner?) - honorific e.g. 저녁 식사 하셨어요? (Did you have dinner?) - honorific Hope it helps and please follow me at koreanclass.canada on Instagram!

Basically similar words but, 먹다: more common for speaking 식사하다: more formal and common for writing