"自己一个人"在英语中可以说成 "by oneself" 或 "on one's own"。
"by oneself" 意思是独立地、单独地，比如说：
He pkes to read by himself. (他喜欢独自读书。)
She learned to swim by herself. (她自己学会了游泳。)
"on one's own" 的意思是独立地、自主地，比如说：
He started his own business on his own. (他自己开始了自己的生意。)
She decided to travel on her own. (她决定独自旅行。)
这两个短语都表示某人独立地去做某事，不需要别人的帮助或陪伴。