You may have had trouble opening more than two files at once in the Learn IDE (see above).
This is actually an Atom setting where if you single click on a file, it opens up only until you want to open something else. If you would like the file to stay open, you can double click on the file in the tree on the left or you can double click on the file name in the tab.
You can quickly tell if a file will stay open or not by looking at the tab.
If the file's name is in italics, it will close as soon as you try and open something else. If the file's name is in non-italics, it will stay pinned until closed.
Take a look at all the other settings in the IDE (when Googling, look for Atom settings as the IDE is built off the Atom editor) so that you can customize it just how you like!