Why is it easier to learn something when you have a teacher explain it? I find that things you learn in school are much easier to learn than before you take the course. For example, there is a mathematical theorem that seems unnerving when I self studied calculus, but the same theorem appeared to be so much easier when I actually learned it in school.

Why is it easier to learn something when you have a teacher explain it?
A bad teacher would explain things in a manner that's harder to digest, when compared to a good teacher. However, in general, for any teacher, even a tutor, it is easier to concentrate because the prefrontal cortex is reminded continuously of the goal (of learning). This is called goal-directed behavior. Learning alone, however, makes it harder to concentrate at the same level. I may have misunderstood what you you were looking for in the answer.