YOU SAID:
I went down to the store to buy milk.
INTO JAPANESE
私は牛乳を買うために店に行きました。
BACK INTO ENGLISH
I went to the store to buy milk.
INTO JAPANESE
私は牛乳を買いに店に行きました。
BACK INTO ENGLISH
I went to the store to buy milk.
You've done this before, haven't you.