when sentences end with '가다' it would be formal sentence. ex) 여행을 가다 (go traveling) , 학교를 가다 (go to school) I don't often use ~가다 but i can see especially on the internet or in the book '~가다' used. I recommend this site: http://www.talktomeinkorean.com/lessons/level-3-lesson-7/ you could learn about 어/아.