YOU SAID:

We went to the store to buy milk and then we saw an old lady

INTO JAPANESE

ミルクを買いに店に行ったら、おばあさんに会いました

BACK INTO ENGLISH

When I went to the store to buy milk, I met my grandmother

INTO JAPANESE

ミルクを買いに店に行った時、祖母に会いました

BACK INTO ENGLISH

I met my grandmother when I went to the store to buy milk

INTO JAPANESE

ミルクを買いに店に行ったときに祖母に会いました

BACK INTO ENGLISH

I met my grandmother when I went to the store to buy milk