'집들이' : a celebratory party after moving house. '미리 장을 봐 두었어요.' : I went to the market and bought them food before the party starts. ~ 해 두었어요. : To make it easier to later behavior was previously. Do you understand?

미리 장을 봤어요. 미리 장을 봐두었어요. it is same thing. only different is nuance. formal form is 장을 보다 -> 장을 봤다. 장을 봤어요. it seems that "장을 봐 두었어요 " is more polite. Have a nice day~

공부하다-공부해두다 공부했다-공부해두었다/공부해뒀다(두었=뒀) 밥하다-밥해두다 밥했다-밥해두었다/밥해뒀다 먹다-먹어두다 먹었다-먹어두었다/먹어뒀다 사다-사두다 샀다-사두었다/사뒀다 장보다-장보아두다/장봐두다 (보아=봐) 장봤다-장봐두었다/장봐뒀다 Upper is present forms. Lower is past forms. Many basic verbs could be transformed into -두다. When 두다 is added, the meaning is changed a little. For example, 장보다 means to buy a lot of food. While 장봐두다 means to finish buying a lot of food. Therefore, -두다 means 'finish-' I hope it will helps ^------^