My sister sings more happily than (I). You study harder than (she).兩句錯在那?than me及than her都是口語用.或than I (do)及than she (does).?

第一句的語意就有點不清： 
如果你想說的是： 我的姐妹比我更享受唱歌。
那應該為：
✔️My sister enjoys singing more than I do.
 ❌而非My sister sings more happily than.➪錯誤的點：happy有比較級happier，happily是副詞；被比較對象要寫出來，且前後比較物要相同

所以第二句應為：
You study harder than she does. 
前後比較物為 對於兩個人學習的態度
如果寫You study harder than she. 前後比較就是比較（你的）學習態度跟一個人（她），而不是你的學習態度跟她的學習態度

Re:-Chen's the heart of grammar:-
"Than" is a conjunction, though ellipsis sometimes makes it seem a preposition. 
If you add the verb in your mind, you won't confuse the case.
RIGHT:-My sister sings more happily (than I am)..(NOT than I do).
RIGHT:- You study harder (than she does)=(than she is.)
"than me&than her" are vernacular and objective  using the native spoken language of a country or area..