Duolingo, Please when you are doing questions like 'six hundred,' make each answer two characters long. If the correct answer is two syllables long and you have three monosyllables next to the actual two-syllable answer ON EVERY QUESTION (ie: zai jian, ni hao, etc), then you are not training us to recognize the answer, you are just training us to tap on the two-syllable one!
Because the alphabeth sounds different in every language. For example, "H" have a "R" sound in english, but in portuguese it doesnt have sound. You must not think about the english sound of the letters but focous in learn it all again in the chinese way. Also, hm, I think it's better to you to listen "liu" carefully, it doesnt sound like "D" at all lol And "B" can seems to sound like "P" some times when you speak it too fast with too much air but the correct sound really is "B"