( )he was ill, he worked hard

Although
Even though
Despite the fact

(Although) he was ill, he worked hard.
尽管他病了，他还是努力工作。

Although though 尽管

although较正式常用于句首
though在此句也行.非正式语体中较普遍
Even though 也行 表示强调
该句没让区别所以三者都行