Miriamise the explanation? ~chuckles~
Bubu, I'm not sure what to tell you. It's sometimes too easy to give advise when we're not involved in the situation ourselves. I don't know why your students made those mistakes. Did they have enough practice before you gave them that activity? In what way/s do you deal with grammar in your classroom?
To many teachers, and perhaps to even more padagogists, "grammar" sounds like a taboo word. To me, on the other hand, grammar is a great tool. What do you do when the problem is grammar? Explaining grammar doesn't necessarily mean that you have to get too technical.
I agree that it's not necessary to "overuse" grammar explanations, but then the grammar of a language is as important as any other aspect of it. Teachers have been, for many years, claiming that communication is the goal, the beginning and the end. Well, in my opinion, in order for a person to communicate effectively in any language, grammar is important.
Many students are taught "fixed" constructions -according to their function- so they will be able to ask "How old are you?" or "Can you tell me if there's a post office near here?", but they will have problems when they have to build their own sentences.
Teachers' taboos only hinder learning. I'm not saying that every single lesson should be based on grammar, but the other extreme isn't "healthy" either.
Sometimes a grammar explanation is the only thing that will work. Sometimes translation will work; sometimes things not favoured by the communicative approach to teaching will work wonderfully. I think we have to use anything that will help our students. It doesn't matter if it goes against this or that approach. The question is "Does it work?". If it does, then it can't be that bad.
Have you tried giving your students the right question to ask, perhaps together with a question along the lines of "Who did the steady stream of fossils discovered... dazzle?". You can make them compare both questions and call their attention to the difference in the form of the verb in each question. That will be working with grammar and still not using specific or complicated terminology.
"What dazzled someone?" contrasted to "Who did something dazzle?" will surely bring up interesting responses from your students.
But, if that doesn't work either, then why not give them a grammar explanation? A simple one if you wish. I can't tell if that's the best thing to do with your group of students, but I think that exposing learners to grammar rules and to grammar terminology can be a plus.
I hope someone with a different view will post a response here, and I'm sorry I can't be of help.
Miriam