немного/несколько - I confuse these all the time and cannot decide clearly which to use when.
немного is literally NOT MANY/MUCH
несколько is several/some. Or to be more clear несколько is some undefined amount.

But make no mistake, you could still say:
я ещё раз написал немного предложений на русском языке
but несколько sounds just much more smoother and authentic in this case

I was about to write проверьте instead of проверку. I could not figure out what grammatic form is проверку. I cannot understand it clearly. That is why I could not make up my mind which to use. So what is the grammatic structure of проверку?
проверку - noun, accusative of проверка.
проверьте - verb, imperative of проверять

Why из тех is needed in the 3rd sentence?
To make the statement more clear. To imply that you mean not any street in the world but the streets which are located in St. Petersburg. But I'm going to second Боб Уайтман that the sentnence is better off if rephrased

Well, I used к Турцие in the 5th sentence because I thought that к Турцие states the seas are located around Turkey. Like they cover around it rather than stating they are in Turkey.
Hmm, I still don't get it completely. What do you mean? That the seas surround Turkey or that they border Turkey, or perhaps that they are located throughout Turkey? Anyway, let's take a look at each of those variants

The seas surround Turkey:

Моря, окружающие Турцию, очень красивые or Моря, которые окружают Турцию, очень красивые.


The seas border Turkey

Моря, граничащие с Турцией, очень красивые or Моря, которые граничат с Турцией, очень красивые.


The seas are located throughout Turkey.

Моря в Турции очень красивые or Моря, (которые раскинулись) раскинувшиеся по всей Турции, очень красивые or Моря по всей Турции очень красивые