But you should know! Otherwise you become one of those imbecile English 'teachers' who take points of for "He was late," because it 'is passive.' That is not in the passive voice. It is not in the active voice. There is no voice. The sentence's predicate is a copula with a predicate adjective ('late').
Grammar explains how the Legos of our language go together, just as structural mechanics explains how a building stays up, or cost accounting explains how a business pays its bills.
Irving Berlin never learned how to read music. Only a genius could succeed as he did without learning the formalities of the craft. But John Williams, an inspired workman, gave us the Star Wars music, using what he learned from predecessors greater than he.