How does one write well?
Whether one is writing fiction or a five-paragraph essay, all good writing is about telling the truth. Even fiction must still hold the essence of truth in order for it to resonate with people in a meaningful way. Truth is more than delivering facts; it is about persuasion, education, and motivation. Writing is about delivering the truth in a new manner which has the ability to change perception, inspire imagination, and demonstrate original and critical thinking.
Do you feel like you have a pretty good handle on the English language if it is written on paper, but when someone tries to engage you in conversation you lose confidence?
Firstly, stop trying to be perfect! Even native speakers often make mistakes and that's OK! Mistakes are proof that you're trying. English is a tricky and complicated language. There are so many rules and complexities that it can be frustrating until you find the underlying structure and become comfortable with its flow. So practice, practice, practice!
Why is it necessary to learn arcane grammar rules which no one seems to follow?
The English language has many functions, including communication, education, developing ideas, and persuasion. English also has many facets, such as vernacular and formal, and it is important to understand the the differences to use the language appropriately. The rules are in place to ensure optimal understanding from speaker to listener or from writer to reader. Understanding English in its correct form will give you the ability to speak eloquently and write proficiently when it matters most. Does that sound a little boring? It won't be so boring when the city council members approve your written proposal for that awesome new skate park in your neighborhood, will it?