(This tip may not be suitable for beginner)
Find something AMUSING or USEFUL to write about.
I know that many members are very supportive but it doesn't mean that they will always be willing to help you with your learning.
Moreover, for some languages (like English), the number of people who need help can exceed easily the number of people who can provide them.
Now let put yourself in their shoes and answer these questions:
1.
Do you love to check this website regularly "just to help" dozen of internet strangers without any reward? 2.
Do you love to read A LOT OF extremely boring articles just to find some grammatical errors so you can fix them?
No reward of course! 3.
Do you love to read something totally unrelated to you while you can spend that time on Reddit to find something funny to read? 4.
And if you're having your OWN problem, will you solve it first or you'll put someone else's need before yours?
In short, make it interesting so others will bother to take a look at your text and fix it.
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P/S: Unlike other forums where they focus solely on ONE language only, websites like Polyglotclub or Lang8 may have more members but lack focus.
The platform is great, though.
(Fixing phrases is more convenient) However, I don't think you'll receive many feedbacks.
My previous post was posted on Reddit (in r/languagelearning) and Lang8 and both received some feedbacks after a few minutes while the one here is still untouched.(But sometimes, it's the opposite) .
So I'd like to give you one extra tip:
SEND YOUR POSTS TO DIFFERENT PLACES INSTEAD OF ONE.
By doing that, there's a big chance that you'll receive feedbacks very soon and you don't have to waste your time waiting then get disappointed.