return back???

雖然有字典指ill 可作名詞, 但illness才是常用來表示"疾病"的名詞,

所以He has a ill seriously. 應改作 "He has a serious illness."

另外, He is seriously ill　和　He is ill seriously 這兩句, 肯定以前一句使用頻率高許多

正如廣東話可以說: (1)佢病得好嚴重呀! (2)佢好嚴重呀病得!
明顯是第(1) 句比較順口 (請留意副詞位置)

學英語自然也應須分辨哪些是常用表達法.

seriously係副詞,可以擺ill前面/後面
He is seriously ill.
He is ill seriously.
He has a ill(名詞) seriously就唔啱,要改成,因為:
1 i係響音,要用an
2 副詞只可形容/修飾動詞,形容詞修飾名詞
He has a serious ill.