a和an的用法,讲解下

1． 表示“一”这个数量概念,比one意思稍弱 e.g.:The plan will be ready in a week or two.2． 表示某人或某物,但不具体说明哪个,相当于any e.g.:This poem was written by a student.注：在1,2两项中,如果名词变复...