I wrote. "Did you see the teacher?" Which is wrong. I'm guessing because its present tense and not past tense... Also a little fuzzy on the used of "den". Why not "der?"
You're right about the present tense. "Den" instead of "der" because it is a masculine (singular) noun, and it is in the accusative case.
Case link: http://coerll.utexas.edu/gg/gr/cas_01.html
Definite article link: http://en.wikipedia.org/wiki/German_declension#Definite_articles.5B1.5D
If this seems overwhelming, my suggestion is to bookmark the pages and refer to them often. With just a little practice (and studying) you'll get the general gist of it and be able to answer some of your own questions. And if not, then just keep asking!
I see! Thank you.
So for two masculine nouns, one the subject, the other the object:
der [subject] VERB den [object].
"der" marks the subject, and den marks the object, so it would follow that the following would have the same meaning (regardless of order of the subject and object, unlike English where order matters):
den [object] VERB der [subject].