在下列各句的空白处填入一个最恰当的单词，使句子意思完整，语法正确。
1. I'm afraid to talk with others in English because my_______English is poor.
2. John ran more_______than the others and missed the train.
3. Be careful not to make the same_______in grammar.
4. It's a_______between you and me. Don't tell others.
5. Taking lots of_______in class is very helpful because you can review well after class.
6. To _______with, she spoke too quickly, and I couldn't understand every word.
