I not writing a letter.改错

I not writing a letter.改错 I am not writing a letter.改错、 I'm writing a letter I am writing a letter. She is not writing a letter.=She not writing a letter. I‘m writing a letter.改成问句 I'm writing a letter now I am writing a letter是什么意思 I am writing a letter.提问： I ( ) writing a letter to Mary 填空 "I" am a letter [改错] writing a letter是什么意思