ADHD can make learning difficult.
A child with the disorder needs to be taught other ways of getting information because he can have trouble staying focused long enough for the facts to sink in. His working memory--how much data he can hold at one time--may also be weak. So even though he can understand vocabulary words, he can have trouble recalling and writing them down fast enough on a test. (Kids with ADHD often get extra time to complete tests.)
Dr. Hallowell takes issue with the very name ADHD: "These kids don't have a deficit of attention; they have a wandering of attention. Like a toddler at a picnic, their brain goes wherever it wants to go." In fact, many can actually become hyper-focused on something they're interested in.