I'm can't figure out how duolingo is distinguishing between when "the" and "a" should be used. What's the trick here?
There's no consistent logic here... so just point it out as a mistake every time :)
A pattern seems to emerge...
The 것은 term at the end of a gerund phrase can be substituted with any noun!
In this case, 것은 is substituted by the word 남자. Now the verb in the phrase is describing the man.