1 我每次旅游大概需要3天时间.
I spent about three days each time I travelled.

2 你上次旅游 是在什么时候?
When did you travel last time?

3 上次 我旅游 还是在3年前
It was three years ago that I travelled last time.

4 上次 我去上海旅游,还是在3年前
It was three years ago that I travelled in Shanghai.

5 你最想去旅行的城市是哪里?
Which city is your favorite for travelling?

6 我最想去旅行的城市是上海.
My favorite city for travelling is Shanghai.It was three years ago that I travelled in Shanghai.请问，为什么用 that...这是个什么从句呀。谢谢