When I go to Nha Trang, I buy differents gifts for my friend in the USA.
When / I / Nha Trang / buy / differents gifts / friends / USA
\(\Rightarrow\)When I was in Nha Trang, I bought different gifts for my friends in the USA.
When / I / Nha Trang / buy / differents gifts / friends / USA
=> When I go to Nha Trang, I will buy many differents gifts for friends from USA
When I went to Nha Trang, I will buy many differents gifts for friends from USA.
