・I have worked 〜と現在完了の時制にした方がより自然だと思う
・workの綴りミス
・とても疲れている(原因)→勉強したくない(結果)
なので This is why〜が正解
【翻訳例】
I have worked for six hours, so I am very tired now.
This is why I don't want to study English hard to pass the entrance exam.