look!this is an“F”(为什么用an)

look!this is an“F”(为什么用an) This is an alarm clock 为什么要用an? Look! This is an _____ .And that is an _____ . &nb this is an orange.it is orange.为什么第一个加an,而第二个不加an Is this an egg? Is this an eye? This is an orange this is an easy this is an 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 为什么This is Ann an that is her mother.谓语动词用is Hey,please look at this picture.this is an English teacher.H