语法上 could you play the violin last year?没有错
而且,加个 already 意思就更精确了：
Could you already play the violin last year?
不过一个比较习惯的表达方式是
When did you start playing the violin?
也希望点击“选为满意答案”及时采纳,可是could you ,,,,好像是委婉语气，但是这里could you表示过去时态，这冲突吗是的， could 有两个用法， 委婉语气是其一，过去式是其二， 所以加 already 。could you already 强调过去式的用法。你也可以说were you already able to play ...但这说有点别扭。