We can say 啊is the most frequently used interjection in Chinese language. If you come to study in China, you should know when it is pronounced in the rising tone, it means to ask further like啊？你说什么？When in the falling-rising tone, it shows being surprised like 啊？怎么会有这种事？When it comes to the falling tone, the short articulation shows to respond like 啊，好吧while the long one means to realize something like 啊，原来是你. It can be also used to express amazement or surprise like啊，伟大的祖国！
Of course, it also can be articulated in soft voice carrying many levels of meanings. First of all, it can be used at the ending of exclamatory sentence to strengthen the tone. You can say多好的天儿啊. When you learn Chinese online free, you may learn it adds certain emotional coloring when being placed at the ending of declarative sentence like 这句话说的是啊.
However, if it is in the end of imperative sentence, the sentence will carry the meaning of supervising and urging and reminding. For example, 你可别告诉小邓啊. It also has the function of decreasing the tone when being placed after the interrogative sentence like 他什么时候来啊？It can be also used in the middle of the sentence to make a short pause so that the listeners to pay attention to the following contents. For instance, 这些年啊，咱们的日子越过越好啦.
You can also often see it is used after the examples listed like the sentence书啊，报啊，杂志啊，摆满了一书架. If it used after the repeated verbs, it indicates the process is really long like乡亲们盼啊，盼啊，终于盼到了这一天. One point that the students who learn Mandarin should know is that 啊is often influenced by the vowels before it when it is placed in the middle or end of the sentence and being pronounced or written differently.