What should I do if my English teacher often makes mistakes in class?

Tell him before class ~ tell him quietly to save face for him

If it's hard to say in class, give him advice in private.

The most important thing is that you should be sincere and tactful.

Of course, if it really doesn't work, write an anonymous letter to your superior and dismiss him.

Finally, this method should be used with caution ~ have you asked other students if they feel the same way?