hello Konan~ i agree with lizi.
"i did my homework", i think "did" just can show that you worked on your homework, but maybe you just did a small part, not really finish it~! so you can't say "我做完作业了",you should say "我做作业了" or "我做了作业". there's a big difference between "做完" and "做了". "做完" means you finished!
hope my answer can help you~! :)