呃:
1. hiccough
2. HAW
3. Ur
4. eh
Examples:
他说："老兄,原来你在这里,你还活着吗,呃?"
"So there you are, old chap, still in the land of the living, eh? " he said.
啊:
1. ah
2. (interj.)
Relative explainations:

Examples:
1. 啊,你在这里呀!
Ah, there you are!
2. 哦!天啊!我英文没有考及格!
Oh, dear! I failed in the English examination!
3. 啊,好啦,一样白米养百样人嘛.
Ah, well, it take all kinds!
4. 啊,我真喜欢这首歌!好高兴!
Ah, I really love this song! I am so glad!
5. 多可爱的婴儿啊!
Isn't that baby a dear?