亚(yà) - Asia (in compounds); sub, second in Mandarin

亚 (yà) does two jobs, and neither of them lets it stand alone. It abbreviates 亚洲 (Asia) inside names such as 东南亚 (South East Asia), 东亚 (East Asia) and 亚运会 (the Asian Games). Separately it means one rank below, which is where 亚军 (runner up) and 亚热带 (subtropical) come from.

The two senses are historically unrelated. The sense of second or lesser is the old native one; the Asia sense is a nineteenth century transcription that reused the character for its sound. Learn them as two entries that happen to share a shape.

The trap is treating 亚 as a free word. You cannot ask 你去过亚吗 about Asia; it has to be 亚洲. And 亚军 is a silver medal, not an Asian army, which is a genuinely common misread for learners who parse 军 as soldier first.