double-bass

I want to learn to play double-bass.→I want to learn to play the double bass.

‘Double-bass’ 是形容词（如 double-bass instrument），指代乐器本身时应使用名词短语 ‘double bass’ 并加定冠词。中文母语者受 ‘低音提琴’ 是名词的影响，误将 ‘double-bass’ 当作名词使用。

She is a double-bass.→She is a double-bass player.

‘Double-bass’ 是形容词，不能直接作表语指代人。需补充名词 ‘player’ 构成完整名词短语。