借题发挥的英文：
[Literal Meaning]
use/subject/develop further in meaning or ideas
to make use of the subject under discussion to put over one's own ideas

[解释]
假借某事为理由，表达自己真正的意思或者去做自己想做的事。

[Explanation]
to seize on one incident as a reason to do another thing

[例子]
本来这事没有什么大不了的，没想到他借题发挥，非要李明道歉。

[Example]
Actually that was nothing at all, but he seized on the opportunity to force Li Ming to apologize.

seize on a theme as a false pretext to express one's own view;Play on the theme of
借题发挥