when i'm serving, my foot accidentally stepped on the line, and my friend said it's a violation to do so. just wondering, is it true that my foot can't touch the line when i'm serving?
Yes, it's a fault. For more details, download a copy of the Laws of Badminton from www.intbadfed.org.
the particular law you have violated is Law 9.1.2 the server and receiver shall stand within diagonally opposite service courts without the boundary lines of these service courts.
Firstly... both of your feet must be within the boundaries of your own service area from the 'start of service' until you (the server) has contacted the shuttle (or missed the shuttle). Secondly... your feet cannot even take a step during service; your feet must remain in the same position from the 'start of service' until the you have contacted the shuttle (or missed it).
nobody spotted the mistake? Law 9.1.2 the server and receiver shall stand within diagonally opposite service courts without touching the boundary lines of these service courts.