"Die Frau trinkt mit dem Lehrer."
Can someone explain why this is dative?
Best tip: Learn by heart (make a poem out of it, or so) "aus, bei, mit, nach, seit, von, zu" (always followed by dative case) and "an, auf, hinter, in, neben, über, unter, vor, zwischen" (followed by dative when answering "wo", "where" z.B. "Das Buch liegt auf dem Tisch") and after a while it comes automatically without going thru lists in your head :)
The preposition 'mit' is ALWAYS used with dative. Always. There are more prepositions used only in dative, like bei for example.
also look at the Tips and notes in this exercise. I think the answer is well explained over there
I wrote teachers instead of teacher. I thought Lehrer could be singular or plural,...?
But you have "dem Lehrer" and it comes from "der Lehrer", which is singular. If you had had teachers, there would be written "die Lehrer" ;)
Actually the first afirmation is correct, but the second do not. In this case the article of Lehrer should be in the dative case, and the dative for plurals is "den". So it should be: dem Lehrer (one teacher) den Lehrer (two+ teachers)
You can take a look here for further information: http://en.wikipedia.org/wiki/German_articles
Because it is dative case, the teacher is taking part of the action and not being the one acted upon. Hope that clarifies a little. (I didn't get really well when to use one case or other yet, but anyway)