Your kind remarks are much appreciated.
Firstly, the best, most obvious, and the easiest, way to improve one's written English and to increase one's vocabulary, and understanding of grammar, syntax, sentence structure, and so on, is to read, read, and read. And then read some more.
Secondly, what you choose to read does not matter an awful lot - comics, books, novels, classics, newspapers, broadsheets, magazines, periodicals, children's stories, - as long as you are reading constantly. Indeed, if you read widely, you will gain an understanding of the different tones - and vocabularies - used in different forms and formats.
Every writer I have ever heard interviewed - or have met - when asked to give advice for budding writers almost invariably recommended that you read endlessly before you can even begin to think of finding your own voice in writing.
Now, for what it is worth, I used to teach history and politics at third level for the best part of two decades.
One of my professors - the then head of department - an excellent boss, with whom I disagreed on this matter - once instructed me that when grading the term essays of the students to ignore their use of language, syntax, grammar and so on. Hs argument was that what was I was examining was their understanding of politics, and political concepts, and that the language in which they chose to attempt to analyse and address this was irrelevant. "You are not grading their understanding of English, but of Politics," he remarked.
I was considerably younger at the time, and had just started working as a teacher in the department. Moreover, I liked and respected him, so I didn't challenge him at the time, because - on the surface - I could see his point.
However, I'd challenge him now. This is because - irrespective of the subject matter, - whether it is Politics or Shakespeare - mastery of a basic competency in English - a mastery that is sufficient to argue, or present, or make your case clearly and cogently - is an absolutely vital skill to have mastered. And the written language is an agreed means of communication. Save your artistry (as in 'I am expressing myself and don't wish to be constrained by the tyranny of grammar' or '"I have the right to use text-speak in essays 'cos thats how i rite'") for poetry or prose you intend to submit as fiction. In a formal essay, slang should be used sparingly.
Essay writing is not just a test of your knowledge of, and understanding of, the topic that has been set. To a far greater extent, it is also designed to test whether you can make an argument, or marshall your material to make a case using the arguments, data and facts that you are supposed to have learned when studying this particular course, or topic. Essay writing teaches you to present material, and weigh up the merits of competing sources. This is why, very often in the Humanities, there is no 'right' answer; rather, there are answers that could have been a lot worse, and some that may have been a bit better.
Now, you may well ask what this has to do with the question originally posed by the OP?
Simply this. In my time teaching, I noticed a gradual decline in the quality of the written English in essays and papers presented by students. When I started teaching, in the late 1980s, the average university student at the age of 18, when they would be starting university, (and there were always those who read widely, - and thus, wrote well, but they are not by any means a majority) had a written vocabulary roughly akin to that of a 15 year old, a mid teen.
Thus, their essays were clunky, a bit awkward, but comprehensible. In general, by the time they left university, their vocabulary had improved to that of an 18-19 year old, which was more than perfectly adequate for their needs.
By the time I quit the groves of academe, and was persuaded to abandon the Ivory Tower for the Real World, I realised that the written vocabulary of the average 18 year old had become closer to that of a twelve year old - i.e. a kid just about to start High School, or second level.
Now, there was nothing wrong with their brains: They understood quite clearly what was going on in class, and got the concepts that we were discussing and disputing. However, they lacked the vocabulary to discuss it at the level required, to be able to interrogate it, or analyse it, in writing or - less frequently - orally.
The vocabulary of a twelve year old - no matter how bright that kid actually is - is insufficient and inadequate to handle adult philosophical, or political concepts in a professional, informed, objective, and critically analytical manner.
Recent functions - book launches, wine tastings - where I have met former colleagues, one or two of my own former teachers, (and one of two of my own former students, who are themselves now academics) tell me that the situation is, if anything, even worse than when I left the academic world.
That the kids are borderline illiterate in some instances. That expecting basic literacy in essays is a dream, that they spend their time teaching kids how to write and structure essays, meaning a beginning, middle and end. They find themselves teaching sentence structure, how to express thoughts on the written page, why paragraphs matter (my former student is now teaching politics and he mentioned that too many of his classes are devoted to basic English writing and comprehension skills - we both laughed at what my former professor - who also taught him one year - would have said), that spelling and grammar have a role too when writing and putting your thoughts on paper.
They were all clear on the causes behind this shocking decline in standard of basic literacy. The kids are not taught how to write, they read next to nothing, they have short attention spans, and they have absorbed the message that they have a right to express themselves howsoever they wish.
Now, back to your question. Read, read, read. This will allow you to absorb - unconsciously - how sentences are structured, and the subtle differences in how individual writers express themselves on paper.
Then, when you find a writer whose style (and content) you like, read that closely (and feel free to worship): I used to revere the writing of - say - Neal Ascherson, or someone such as Conor Cruise O'Brien, or Timothy Garton Ash.
These were - and are, both Garton Ash and Ascherson are still alive - writers whose supple, original and informed minds are well worth reading simply for intelligent and thoughtful content and whose mastery of their craft as prose stylists who love language makes reading them an intellectual - and viscerally physical - pleasure. (That doesn't mean that I always agree with their conclusions: I don't. But I would never miss an article of theirs because that fusion of a first rate mind allied with a sublime pen makes for literary perfection - to my mind, at least).
Anyway, my point is to find someone whose writing - whose prose style - you like, and read it closely. You don't wish to channel them or to try to clone them (which of us would channel Oscar Wilde, even if we could? We are not him, and have not lived his life experiences brilliantly gifted though he was). See what you admire in that, and see how that fits with how you express yourself. And read, read, read. With a curious, critical, and yes, admiring eye. And an eye open to just enjoying the pleasure of reading.
Good luck.
