Steemit has helped me improve my English. If your post has poor Enligsh, it will look like you didn't spend much time on it. English is not my first language, as I'm from Norway, but I'm very interested in language and that's why I went to school to become a journalist. My Norwegian is of course a lot better than my English. I'm not saying my English is very good. It has just improved a lot.
Here is what I have found helpful in improving my written English:
1. Write
And write, and then write some more. How do you get better at running? By running of course.
I can see a big difference in my English when I look at my first Steemit post
2. Read English books
I have always read a lot of books. But when I started to write on Steemit, I decided to only read English books. I downloaded a whole lot of books on my Kindle. All this reading in English has definitely expanded my vocabulary. On the Kindle I have the option to look up words that I don't understand. I do that with every word that I haven't heard before. I can probably only remember 2% of them, but that's 2% more words than I knew before.
3. Proofread
I usually double-check my writing. The first time I read through it, I look for long and weird sentences, the second time I look for spelling mistakes and weird grammar. It's also helpful to read it out loud the second time. I look up words in a dictionary or a digital dictionary if I'm not sure about the spelling. All this proofreading has actually helped my written English because I remember the correct spelling after a couple of times of looking it up.
4. Read articles about the subject you'd like to write about
I like to write about traveling. I read tons of travel magazines and web sites about traveling. This way I can pick up on words that are relevant for that topic, and even pick up on trends or exciting new destinations.
5. Get the opinion of a friend
As a journalist, I'm lucky to know other journalists that can give me a second opinion on my writing.
Find a friend that also enjoys writing and give each other constructive feedback. This is a great way to discover if you have any bad writing habits.
6. Take a writing class
I've never done it, but I would love to try it some time. This is alsa good for feedback on your writing.
Some exciting news:
My friend, 
Susanne