This is a topic that's also reeeeally difficult for native German speakers. And to be quite frank most people don't take it to seriously. But as a basic rule: You CAN almost always use a comma in this case, but don't have too. "Es ist leicht, Dich zu verstehen" is as correct as "Es ist leicht Dich zu verstehen". But! If the verb is "brauchen", "pflegen" or "scheinen" you are not allowed to use comma: "Es scheint gut zu sein" (not "Es scheint, gut zu sein"). There are some other special rules (which really rarely apply) and you can check them here: http://www.studis-online.de/Studieren/Richtig_schreiben/komma_bei_infinitivgruppen.php Even if you don't understand most of the German, the most important rules can be found in the grey outlined boxes :)