i( )a letter today,but o far i ( ) it答案是have been writing;haven't finished,是因为so far用have吗?为什么前面的不能用have wirtten?

Boss,must I finish typing this letter today?No,you__.You may finish it tomorrow.A.needn't B.mustn't C.don't D.won't Today is a red letter day “I”（ ） a letter “I”（ ）a letter But I () a letter I ( ) a letter How are (your) today?''I'' (am)a letter?Your friend （are） studendly Andy and lily (is) StudentsHow are (your) today?''I'' (am)a letter?Your friend （are） studendly Andy and lily (is) Students 英语 I learnt a lot from__（today）talk It was nice（）you（）me the letter i wrote a letter.passive I'm writing a letter I am a letter是什么意思 I am a letter [改错] A:I have read today's newspaper.B:( )( )my father It 's( )today.I canim.a:sunnyandhot b:cIt 's( )today.I canim.a:sunnyandhot b:cloudyandcold c:readabook It 's( )today.I canim.a:sunnyandhot b:cIt 's( )today.I canim.a:sunnyandhot b:cloudyandcold c:readabook today is ___,i have computer today.A:Thursdays B:Sunday C:Thursday 选什么,为什么 寻一首关于英语音标的英语儿歌,部分歌词是这样的I know the letter a says A and a,I know theI know the letter a says A and a,I know the letter b says bbb,I know the letter c says kkk,I know my alphabet,I know the letter d says ddd,I I'm ____to receive a letter soon A.Expecting B.waiting 答案选哪一个,I'm ____to receive a letter soonA.Expecting B.waiting