Obviously the last two reasons seem more like excuses. Native teachers who worry that their explicit knowledge of English isn’t up to scratch are probably more concerned about saying why a student’s utterance is an error than that they won’t notice the mistake in the first place; and it’s important to realise that (1) you often don’t have to say why something is wrong, as long as you help your students improve (see below); and (2) your explicit knowledge of English grammar will dramatically increase even just by reading through the grammar explanations in your students’ coursebooks. Discovering that you may well not be required to provide an explanation for an error should reduce stress, and this should make it less worrying to give correction even if you’re tired. Non-native teachers often have a great deal of know-how (both of the structure of English and of methods that helped them learn the language) which they can share with their students; they can also help their students conduct their own investigations into what is and isn’t natural English, when they are not sure.