신다 is the verb for putting on anything worn on the feet — shoes, socks, flippers — where clothes take 입다 and hats take 쓰다. -아/어서 gives the reason or the step before: 추워서 신은 양말 'socks put on because it was cold', 너 운동화라도 하나 사서 신어 'go buy yourself some trainers and wear them', 오리발은 물에 들어가서 신으실 거라서 'you'll put the flippers on once you're in the water'.