Look at all the different rules all coming together in that one example! We’ve got an implied subject, a possessive, an object and a past tense irregular verb (“suru” to “shimasen” to “shimasen deshita”). Sure, it may not seem like a big deal to be able to say “I didn’t do my homework” but the amount of grammar represented in that one idea is actually pretty impressive. Good job reader for keeping up so far!
宿題 = しゅくだい = homework
I Didn’t Do It
Blue: To make a polite negative past tense verb you start with the polite negative present tense.
Yellow: That’s just switching ます to ません.
Blue: Then you add でした to the end.
Yellow: Isn’t that the past tense of です?
Blue: Yes it is.
Yellow: So I could say 私の宿題をしませんでした
Blue: I knew you were going to come up with an example like that.
Yellow: That reminds me, can I borrow your math homework for a few minutes?