“Here is a useful book.”当中为什么要用定冠词a,请详细说明.

“Here is a useful book.”当中为什么要用定冠词a,请详细说明. _______horse is a useful animal . 改错 there are sixty minutes in a hour here is an useful book there is cloud in the sky ( )_Hello,is this seat _Yes,you may sit here.A.free B.empty C.useful D.possible A house is a useful animal 翻译 A dog is a useful a_____ O_____ ,it is a useful book _____computer is a useful machine.加冠词? English is a useful tool.写作 The computer is a useful machine. The book is ___than that one.A.useful B.the most useful C.much more useful D.a little usefulThe book is ___than that one.A.useful B.the most useful C.much more useful D.a little useful Here Is A Here Is A Heart It is --- for students to learn English well.Which word can not be used here?A useful B important c necessary D impossible为什么是D What do you think_____ A the most useful invention is B is the most useful invention a useful wood is one of ( )in our life A、useful materials B、more useful material C、the most usefulwood is one of ( )in our life A、useful materials B、more useful materialsC、the most useful materialsD、the most useful material Here is A Hear Here is a book___you.