"Why Can't They Speak and Write Clearly?"
Many students enter college with amazingly poor writing ability, owing to the fact that no one paid much attention to their writing while they were in their K-12 years. Once I had a student come to my office with her test in hand, a test on which she had scored very poorly on all three of the essay questions. “But I never had to write essay answers before,” she complained. Throughout her previous years of schooling, she had taken almost nothing but true-false and multiple-choice tests.
Sounds familiar. I'd just add one tidbit to George's fine article. I found that most students were extremely sensitive to constructive criticism of their writing. As Deirdre McCloskey pointed out years ago, if an instructor criticizes a student's math or economics they won't like it, but most of them will reluctantly acknowledge that that's why they're in college: to improve their skills. But students react to criticism of their writing, Ms. McCloskey wrote, "the way they react to hostile remarks about their weight." (I think she also claimed they took writing criticism similarly to saying they had bad body odor, but I can't locate that.)
Needless to say, this attitude greatly complicates the instructor's attempts to improve their writing.