have an axe to grind

tener una espina clavada, tener intereses personales

• She clearly has an axe to grind — every comment is a complaint.
• He joined the committee because he has an axe to grind about local taxes.
• Be careful with her advice; she might have an axe to grind.

This idiom is used to describe someone with a hidden motive or personal grievance. It's common in both formal and informal contexts, and can be used in both speech and writing. Often employed when suspecting someone of having ulterior motives.

A common mistake is to incorrectly use a different object, like 'a knife to sharpen', which is not an English expression. This literal translation misleads about the intended meaning of holding a personal grievance.

• stand the test of time
  resistir el paso del tiempo
• by the skin of your teeth
  por los pelos, por un pelo
• play it by ear
  improvisar sobre la marcha
• the proof of the pudding is in the eating
  el movimiento se demuestra andando