My pleasure.

Thank you very much for all the explanation, it's very helpful :)

けど大変むずかしい（むずかしい→むずかしい。You should finish the first sentence with "｡" here because you cannot combine the two parts with "だって" even if you seperate them with "､".）だって十分に単語を知りません（知りません→ When you begin a sentence with "だって" and tell the reason for something, you usually finish the sentence with "から". You say "知りませんから" here.）／けど大変むずかしい。だって十分に単語を知りませんから。

日記を書く（書く→"は" is a particle that makes a subject with a noun. So, you should change "日記を書く" into a noun phrase such as "日記を書くこと" or "日記を書くの".）は言語能力を向上する（向上する→”を” is a particle that comes after a noun and makes an object. So, you should change "向上する" into a transitive verb "向上させる".）良い方法です（です→ Even if you use the so-called "です・ます" style, you usually don't use it in this quoted part. You say "である" or "だ" instead.）と良く（良く→ It is absolutely correct, though you usually write it in hiragana like "よく".）聞きました（聞きました→ You can say that, though you probably more often say "聞きます".）。／日記を書くことは言語能力を向上させる良い方法であるとよく聞きます。〔日記を書くのは言語能力を向上させる良い方法だとよく聞きます。〕