Good, but there's a few things that need fixing.
This has an excellent concept, but there are a few things you could improve.
First off, even if you don't make any words, you still go to the next round. There should probably be a point limit you should reach before continuing.
Second, you can fix the plural/past tense problem yourself. Just find the words that you haven't done the plural/past tense for, then put them in this game's dictionary. Third, extra difficulties might be nice. Though, if you can't do that, that's fine.
Overall, this was an excellent game, but it had a few errors.