用所给单词的适当形式填空：
My brother______（study）in Yuhong Primary School when he was 10 years old.
Last month I ______（learn）how to swim.
We must______（learn）from the great people.
（ ）My mother tells me to practice______the piano for two hours a day.
A.to play B.play C.playing D.played
（ ）I'm sorry to keep you_______for such a long time.
A.to wait B.waiting C.waited D.wait
最好能解释、分析原因.