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.