I understand that I can use duolingo off line. I hv tried using duolingo on my ISO devices off line but hv been unsuccessful. Is there a particular step that I am missing? Please advise. Thanks
Unless this is a new feature being A/B tested, you cannot use Duolingo offline. Even if you disconnect in the middle of a lesson, the app will still need to connect to check any answer that doesn't match the model answer.
What is your ISO devices? Do you mean iOS (iPad/iPhone)? When you open a lesson, the app downloads all the questions and acceptable answers for all the lessons in that skill and also all the lessons in the other skills in the current row. I think you have to start one of the lessons while you are online and then you can do the other lessons offline.
Remember though that you have to go on line in order to have your progress synchronised, otherwise you could lose your streak.