It's a must to have a good knowledge of English nowadays.(must前面为什么要用a)

must - n.Something that is absolutely required or indispensable:举例：Promptness on the job is a must.Comfortable boots are a must when going on a hike.must 在这里不是助词,是名词,意思是必须具备的,不可...加a和不加a有什么区别吗？当做必需品名词使用时一定要加 a。如果不加a，可能是另外一种用法。 a new book that is must reading;一本必读书，在这里不加a，因为它作为名词修饰reading. 加不加a与reading 有关，与must无关。 加了a，must 就是主体，不加a，它就是修饰别的词。