1. The sentence 你是从哪里来的？ uses the 是…的 construction. This construction is used in questions where a lot of information is required. So in this example, the speaker may not only be interested in where the person is from, but also why the person came, how long they plan to stay, their purpose for coming etc.

This question is answered (like most questions in Chinese) by replacing the question word with the answer, giving us:

我是从中国来的.

2. As described in the lesson, China is divided into different 省. Other countries, such as America use 州 which are smaller. Taiwan uses 县 which are smaller yet.

3. As described in the last lesson, 已经 is used when describing long periods of time as in 洛阳的历史已经很久了.