You have to add every train use use to your pass. You don't need to plan that in advance, you can do that just before boarding a train.

Yes, you need to enter all trains that you will be travelling with as journeys in your pass, not as trips. You can only have one Trip connected to your pass. To this Trip you connect all trains as journeys. 

Thank you very much for all of the responses.  According to rvdborgt,s response, I can add a new train in the afternoon to the same travel day that I add a train in the morning.  Is it correct? So the travel day is used as one day for a train in the morning  and a train in the afternoon. Am I right? I plan to buy eurail global flexible pass and it is the first time for me to use the app. I am not used to any app and quite worry to use the app. So I just want to make sure that I do the right thing. I am so grateful for all of the responses. 

Yes that's correct.

On a travel day, you can board as many trains as you like, as long as you add the trains you use to your pass. When you add the first train of the day, the app will warn you that you're about to activate a travel day.

Thank you very much for your response. I am now a bit confident that I can use the app. I am so grateful  for this. Another enquiry, Does the app work in an iPad?