but为并列连词。并列连词是连接两个独立句的，though和although引导的是让步状语从句，所以but不能与though和although连用。

错句：Although/Though he was worn out, but he kept on working.

虽然他已经精疲力竭了，但仍然继续工作。

错句：Although/Though he is very old, but he is quite strong.

他虽然年纪大了，身体还很健壮。

例如：

正确：Although/Though he was worn out, (still) he kept on working.

虽然他已经精疲力竭了，但仍然继续工作。

正确：Although/Though he is very old, (yet) he is quite strong.

他虽然年纪大了，身体还很健壮。

英语语法很严格，词义相同但词性不同的不能相互替换。

不能与连词but连用的词有连词though,although是虽然,尽管的意思。