为什么说This is my a pen是错的?

为什么说This is my a pen是错的? 为什么不能说 This is my a pen this is a my pen 新概念英语 不能说my this a pen?要说 my pen?为什么? what's this?It's a pen.为什么不能说是this is a pen.我们老师说就是不能说this is a pen.为什么 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? 我的这一支钢笔 不能说my this a pen?要说 my pen? A:that's your pen.___ is my pen B:Do you __ this is interesting? 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 yours pen.为什么? This is not my pen.M______ is This is a nice pen. This pen is ________and that is ________ pen.A.mine,hers B.my,her C.my,hers D.mine,her this is my pen .改为意思相同的句子 this is a（ ） This litter is ( )my pen friend Tony this is my pen 改为疑问句 this letter is from my pen p___