Grammar: Language Evolves and So Should We

I haven't had a grammar lesson since, probably, the 10th grade. These grammar assignments are really starting to make me second guess my, supposedly strong, grasp on the English language. In Spanish class we're learning about the different conjugations of verbs and it's so much simpler than English! What's frustrating is that the way we speak in day to day life, according to my textbooks, is inaccurate. If our verbal language has evolved to the point where we need not worry about the different uses of lie, lay, lain, laying, blah...why do these professors/scholars insist we learn and use them?