this is 后加"a pen "还是"pen ",为什么"this is paper"可以不加?"these are scissors"可以不加?请说明一下冠词应在什么情况下加吧,

this is 后加a pen 还是pen ,为什么this is paper可以不加?these are scissors可以不加?请说明一下冠词应在什么情况下加吧, This is my a pen?对还是This is a my pen?对This is my pen？ What is this 是This is a pen.还是.It is a pen?为什么? what is that in English?是It is a pen还是This si a pen还是That is a pen This is a pencil and a pen还是These are a pencil and a pen what is this It is a pen.还是This is a pen好这个问句碰到选项,该选哪句? this is a my pen This is a nice pen. 是This is my blue pen 还是This is my a blue pen有助于回答者给出准确的答案 This is a pen.(this)are pencily This is a pencil and a pen还是This are a pencil and a pen,二楼说的也许对，但我只问用IS还是ARE This a pen.that is a pen,什么 This is a pen.就划线提问 this is a pen.改为复数句 this is a pen改成一般疑问句 This is a pen.改为一般疑问句 is this a youe pen 怎样改错 This is a nice pen.的问句