I don't like to criticize another teacher. And there's such a wide variety of types of English that something that sounds unnatural to me could be perfectly normal in another part of the world. But I think there's a problem is with the word order of the sentence "I couldn't teach... only today..". It would be better to say something like "It was only today that I couldn't teach you..." or "I have only missed one lesson..." or " I know it's only today's lesson that I missed.." Is that the part you were asking about? I guess if the person was already having a bad day you could understand why they might make a mistake

So you as a American don't understand it?