Found the Browse All Lessons page, and I see the "Hide Lessons Completed" button, so that addresses this request (though you might want to change the text to "Hide Completed Lessons")
However, I'm seeing a few bugs on the page, and I have more suggestions
Suggestions:
1) The icons are VERY hard to read. How about simplifying them, and using colors that have higher contrast. Maybe just use letters. The red/green Not Completed/Completed icon could be improved just by making it larger -- so you can easily see red vs green.
2) Boost the default "Rows per page" to 50. 5 is just too few. Better yet, Just get rid of it. Personally, I would always have it set to 50. The screen will be simpler without it.
3) Put the "Save Filters" button up a row next to "Select All" and "Select None", because these three buttons are all related. Better yet, put all three of these buttons in the gray section above, delineated somehow, since they are modifiers of the filter buttons.
BUGS:
1) If I boost the "Rows per page" setting to 10/20/50, it seems to still just only display 5 rows -- even though the coloration (black text) indicates my new selection.
2) If I change my lesson filters, the "Rows per page" gets reset to 5. I would expect it to stick with my latest selection.
3) If I select (e.g.) 20 Rows per page, and click one of the Select All/Select None/Save Filters buttons, Rows per page gets (visually) reset to 5.
4) If you're keeping the Rows per page, I would want a way to retain my selection from session to session.
5) Hide/Show Lessons Completed not working correctly. For instance, there are 4 Newbie lessons, and I have marked 2 of them Completed. If I click Hide Lessons Completed, my list is emptied. I would expect to see the 2 uncompleted lessons displayed. I haven't marked any of the Survival lessons completed. If I Hide Completed Lessons, I see a list of 1. If I Show Completed Lessons, then I see all 3.