根据句意，用括号内所给单词的适当形式填空。

1. I think ________(speak) English is more difficult than written English.

2. She is a quick _________(learn), and her English has got better day by day.

3. Be more careful! Don't make any ________(mistake) in your writing.

4. I can't follow what you are saying. Could you please speak more ________(slow).

5. The man laughed when he _________(realize) what had happened.

说明杂质能级以及电离能的物理意义。为什么受主、施主能级分别位于价带之上或导带之下，而且电离能的数值较小？