where has this letter come from能把has换成is吗?两者有什么区别?

in which month does helen write this lette? where in city he has been this 连词成句 who are you going to have(type)this lette for you?不理解这里的时态 where has this letter come from能把has换成is吗?两者有什么区别? 填空－1.I am going to ____(type)this lette.2.my mum can ___ (swim) in a river. this disparity has become even greater— to the point where it is now frequently said, This is the factory___his father has worked for 20 years.空中填什么?(where,which,that) this is where it can get tricky.Each department has a list of courses that are major requirements.this is where it can get tricky. Where has she been? 1.This is a kind of colir.Many giris like it.Some old women like it,too.The word begins with the letter.What is it?Is_______ Where does this stand? When and where this took place ____ still unknown.A：has B：is C：were D：are Where is Jim?I'm afraid he has f___ that we'll have a meeting this afternoon Where is Mike?I'm afarid he has a f_______ .we'll have a meeting this evening China has provided more profit where this scientist is concerned than any other countrieswhere sb is concerned?意思? This company has now introuduced a policy where pay rises are related to performance at work .这句话中为什么要用 where不用which This is the town__he has lived for more than ten yeas.It has changed a lot (which where that)选什么呢为什么不选which where is Jim?I'm afraid he has ____( 忘记）he has to attend a meeting this afternoon.该怎么填