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.