can forms be filled in offline and then submitted when online?

That should be possible if you are using the fillable PDF forms, you can learn more about it on this guide: http://www.jotform.com/help/312-Fillable-PDF-Everything-you-need-to-know-about-JotForm-Fillable-PDF-Creator. Your users can save the changes on the PDF form and submit it when they have internet connection. However, the fillable PDF forms have limitations like widgets, apps and conditions will not work on them. 

If you are referring to the online forms, it is not possible to make them work in offline mode, the scripts and other resources needs an active internet connection for the whole form to work. 

I hope that helps.