Early Birds, Worms, And The Getting Of Them

Here's an early prompt for the week.

What is the grammar principle/concept you have the most difficulty with?  Which/that?  Who/whom?  The Oxford comma?  How do you work it out?

And on a completely different subject, here is a blog post by an English professor in which she describes the process of helping students conduct research projects. I thought you all might find it interesting.