Personally, I have to say I am not happy with the gradual decline in the number of posts and the fact that it's mostly the same people posting or correcting.

They seem to be very close-nit and enjoy interacting with each other within their friend group, which can be discouraging for newcomers who feel left out.

2 Secondly, what's even worse thing is that some memebers are very self-centered.

3 Thirdly, there are some people who only correct others' posts without posting their own.

I would like to tell them, "Don't pretend to be a teacher, y. You need to write in your target language and make mistakes yourself."

Learning for free is never be an appropriate option, so please pay some fees to the developer, Exis-san, if you are a serious user of Nyan-8.

Otherwise, I recommend that Exis-san charge all users a fee.

Personally, I have to say I am not happy with the gradual decline in the number of posts, and the fact that it's mostly the same people posting or correcting.

You just forgot the word "the" between "in" and "number." :P

Specifically, there arI have four points to share below.

Just changed it so it could sound a little more natural. Don't worry, it was grammatically correct before :) It's just for flow.

1 Firstly, my guess is that all current members may behave been heavy users of Lang-8 and probably were friends from back then.

They seem to be very close-knit and enjoy interacting each othermainly within their friend group, which can be discouraging for newcomers who feel left out.

2 Secondly, what'’s even worse thing is that some memebers are very self-centered.

I just took the word "thing" out since it's not needed. And fixed a spelling error :)

Such selfish behavior iscan be contagious to others, leading to a decrease in people willing to correctors.

I changed it to "can be" to soften the tone a bit :) The other changes were just for flow, so that it can sound a little more natural.

3. Thirdly, there are some people who only correct others'’ posts without posting their ownanything themselves.

Added an apostrophe and fixed a typo

I would like to tell them, "To them, I’d like to say: “Don'’t pretend to be a teacher, y. You need to write in your target language and make mistakes yourself." too.”

Your sentence was perfect! I just rephrased it so it could sound a little smoother :P

Learning for free is never be an appropriateot always a realistic option, so please pay someconsider paying a fee to the developer, Exis-san, if you a’re a serious user of Nyan-8.

Otherwise, I recommend that Exis-san charge all users a fee.

Personally, I have to say I am not happy with the gradual decline in the number of posts and the fact that it's mostly the same people posting or correcting.

1 Firstly, my guess is that all current members may behave been heavy users of Lang-8 and probably friends from back then.

They seem to be very close-knit and enjoy interacting each other(only) within their friend group, which can be discouraging for newcomers who feel left out.

close knit, like how when you knit fabric, the stitches are very close together.
"nit" = lice, like bugs in your hair :)

2 Secondly, what's even worse thing is that some memebers are very self-centered.

3 Thirdly, there are some people who only correct others' posts without posting their own.

Learning for free is never be an appropriate option, so please pay some fee to the developer, Exis-san, if you are a serious user of Nyan-8.

Personally, I have to say I athat I'm not happy with the gradual decline in the number of posts and the fact that it's mostly the same people posting or correcting.

This was correct! However, this would make the sentence even more natural-sounding.

Specifically, there arI have four points on this below.

Also technically correct! This is the same feedback, where this sentence would sound a bit more natural with this wording.

1 Firstly, m: My guess is that all current members may be heavy users of Lang-8 and are probably still friends from back then.

If you are listing numbered points, there is no need to use numbers and words that have the same meaning. 例えば、「一」と　「まず」。

They seem to be very close-knit and enjoy interacting with each other within their friend group, which can be discouraging for newcomers whand cause them to feel left out.

2 Secondly, w: What's even more worse thing is that some memebers are very self-centered.

If you look at their profiles and compare their number of posts andwith their number of corrections, you will understand.

Such selfish behavior like this is contagious to others, leading to a decrease in correctors.

More natural-sounding. The previous sentence is still correct.

3: Thirdy, there are some people who only correct others' posts without posting their own.

I wonder whetherif they can understand the feelings of the person writing the article.

"whether" also works!

I would like to tell them, "Don't pretend to be a teacher,; you need to write in your target language and make mistakes yourself."

4: The last one is the issue of cost.

Learning for free is never be an appropriate option, so please pay some fee toshouldn't be taken for granted, so please financially support the developer, Exis-san, if you are a serious user of Nyan-8.

I wanted to make sure your original meaning was preserved. I hope I did not make a mistake!

Otherwise, I recommend that Exis-san charges all users a fee for using the service.