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?

在单数可数的情况下都可以加 |"a",或者"one"...例：this is a pen.this is a pen.在形容不可数名词的情况下定冠词可省略,即不可加“a”与任何表示数量的词例：this is wateror:this is the water在复数的...