"The man is eating with the teacher."
Translation:Der Mann isst mit dem Lehrer.
'isst' already means 'is eating', so what you said there was "The man is is eating with the teacher." Verbs would all be translated along the lines of 'is eating' or 'eats'. Fixes, is fixing; drinks, is drinking; etc. Either one is a correct translation, but putting ist in front doesn't make sense.