He can be very obstinate when he wants to be!

他的犟劲儿一上来，简直执拗得要命！

《牛津词典》

He is very obstinate; you won't be able to make him change his mind.

他这个人脾气犟，你可拗不过他。

《新英汉大辞典》

But our visitor was very obstinate.

但是我们这位贵客实在太固执。

youdao