a和an都是不定代词，表示泛指的一个。a用于以辅音字母开头的可数名词单数前，比如一本书a book，an用于元音字母开头的可数名词单数前,比如一个苹果an apple。

例句：The project is still causing him a lot of problems. 这项工程现在仍然给他带来许多的麻烦。The need to communicate is a key characteristic of human society.需要交流是人类社会最重要的一个特征。An accurate diagnosis was made after a series of tests.准确的诊断是在一系列的检查后作出的。