The first notebook in italki!

Hi HyunJu! You're off to a great start! 

The reason you think that sentence (Clause B) is incorrect is because it is a dependent clause that is not attached to an independent clause. Basically, it is not a full sentence. You can make it correct by attaching the dependent clause (Clause B) to the independent clause (Clause A). It should then say: "I really want to improve my English because I know how to speak..." 

"Worst" is a superlative. It should not be used with the word "than" because "than" indicates comparison. You should instead use the comparative "worse".

"To write" is an infinitive. It is a verb not attached to a clause, therefore not requiring conjugation. However, you used the infinitive in a case where a progressive verb (an -ing verb) would be correct. Therefore, "to write" should be "writing" because it was an ongoing/progressive action at the time.

The first notebook in italki!

The first notebook in italki!

 I really want to improve my English. I know how to speak and make a conversation with English speakers, but I know that I am not actually speaking correctly. (I think this sentence has incorrect grammar)
I mean the reason why I want to improve my English is that I make a lot of mistakes whenever I speak and write.

My grammar is especially poor, so therefore, writing for me is harder, and even worse, than speaking.
My friends told me about this website and I joined 20 days ago, but this is my first time to actually write and post something.
Am I too bad of a student? Ha-ha!
From now on, I will try to write every day or, at least, more than 4 times per week.

I really need and would appreciate your help!

Oh, by the way (or “btw”), I am a Korean, so I can help you with learning / to learn Korean.

Good job! No, you are not a bad student as I understood everything that you wanted to convey. As your vocabulary improves and increases, you’ll be able to compose better sentences and paragraphs, so do some reading in English to expose yourself to it. Finally, watch your tenses. Keep writing and posting! I hope this helps. J.