Dem Lehrer and Den Lehrer? When to use Dem and when to use Den??
German has grammatical cases, like Latin, meaning that you express the subjects relation to the object by changing the object. As seen from your example, der Lehrer in nominativ goes to den Lehrer in akusativ. So if you say "I see the teacher" you say it in german like "Ich sehe den Lehrer". In dativ that der goes to dem so if you say "I give the book to the teacher" its like "Ich gebe das Buch zu dem Lehrer" in german. Here one would also write "zum" instead of "zu dem".