mi is used when you are asking about the subject, mit when you are asking about the object.
Here, you're asking about the object -- "The man does ...." -- the "..." that you are asking about is the object of "does", the thing which gets done by the man.
It's in the accusative case, if you want to be grammatical. "-t" seems to be a generic accusative ending for both singular and plural, which makes it somewhat easier than German and certainly easier than Latin.