this is an apple中为什么要用不定冠词不用可不可以,能不能写成This is apple

this is an apple中为什么要用不定冠词不用可不可以,能不能写成This is apple is this an apple an orange__an egg This is an apple就an apple部分转换提问 this is an apple.改成复数形式 This is an apple 变为复数句子 an,is,apple,this连词成句 this is an apple的an为什么不可以是a This is an orange. There is apple on the table. WThis is an orange.There is apple on the table.What are you doing?译中 用Is this 提问的选择疑问句为什么用It is 回答,而不能用This is回答例如:Is this an apple or a banana?为什么不能用This is an apple.回答?讲出道理! this is an apple 和 this is apple 哪个语法是正确的 “There is an apple”（这是一棵苹果树）中为什么用“an”? These are an apple and a pear.还是This is an apple and a pear. this is an apple怎么改为复数句 This is （an apple）对括号部分提问 This is an apple,that's a knife句型转换 This is (an apple)打括号的地方提问 This is an red apple.(哪里错了) This is an red apple.写出正确的句子