I usually mark as I don't know if I need to look at the options to figure it out.
I use the 'hide answers' setting so I can think about what it is before I see tye multiple choice answers. It helps me a lot.
But ultimately, it is up to you. I mark as 'I don't know' because I don't want to up my mastery on something I don't really know yet. It is just about how you prefer to learn.
Personally, it would be rare for me to mark an "I don't know" on the question. Especially when it comes to grammar, I try to make deduction work for me, but I also try to rationalize my response for my answer. My schedules show my error afterwards and then I can choose what to do with it after. As long as I get it right and remember why it's correct, then I'd have learned.
It's truly your prerogative. If it's not something you're quite ready for, mark it. Otherwise, if you don't try, you don't learn.
I was thinking about that too and I came to the conclusion that marking the wrong answer will reduce your mastery on the wrong term so when you cant deduce even one you should definitely chose "I dont know"
