Take a look at the sentence again:
When Minori left Japan to study in the United States,
she thought that she would miss her mother's delicious home cooking,
however, after her first taste of a bacon cheeseburger, she didn't care
if she ever returned to Japan.
When Minori left Japan to study in the United States is a subordinate clause. A subordinate clause cannot stand alone as a complete sentence. She thought is a main clause because it has both a subject, she, and a verb, thought. When a subordinate clause introduces a main clause, you need to connect the two with a comma.
Go back to the sentence to try again.