On a more serious note, I'll add my vote for 'the'.
It's not just difficult for learners.
I like to think I am moderately proficient in the language, and know something about its grammar, but in my final teaching year I did a lot of work with (very) advanced learners. Part of our time was spent on correcting manuals and reports they had written and on discussing their translation work.
Trying to work out whether 'the' could be left out or not - and why - had me nearly tearing my hair out sometimes.