Did you finish doing your homework中do为什么加ing,did后不是加动词原形吗

finish doing something.
Did 后面动词原形 就是 finishDid 后面动词原形只能跟一个吗？Did you forget to do your homework?这里 后面就有两个动词原形 forget和 do看你如何理解了， forget to do something 毕竟是个固定短语。Did you finish doing your homework中doing在这里是不是动名词是的。