YOU SAID:

I'm going to the sandwich shop to pick up some biscuits

INTO JAPANESE

サンドイッチ店に行ってビスケットを買いに行きます

BACK INTO ENGLISH

I go to a sandwich shop to buy biscuits

INTO JAPANESE

私はビスケットを買うためにサンドイッチ店に行きます

BACK INTO ENGLISH

I go to a sandwich shop to buy biscuits