When reviewing a project in a certain language it's usually convenient to have the original file (usually English) beside the current file for reference.

If you have such file and you want to see it as a 3rd column in the table, open the "Import/Export" menu and click on "Import master file":

That will show you a subtitles import dialog that will add the master file to the project.

If you want to show or hide the master column click on the following icon:

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