Actually,your question also confuses many Chinese people. People from different parts of China may have different tones,some are used to lift their tone while others do the converse way.Generally speaking ,“呢”and “吗”are usually used in the end of a question and followed by “?”。The two words show the doubt about something."啊""吧",these two words are interjections,but "啊 will show somebody is more pleased with something or somebody than"吧"。"哦"just mean "I konw it",it is the reply of a question.And I think if you speak Chinese a lot ,you will soon get it.