Not just Present/Past/Future as a note:
Present Simple
Present Continuous
Present Perfect
Preterite
Imperfect
Past continuous
Conditional
Past Perfect
Future
Future Perfect
Some languages also have the simple future and the subjunctive.
I'm not sure which ones German features, but that's a list of all the different tenses that generally have different conjugations. As I said, it's really helpful to learn them all at once.
edit: Actually just read the wiki article, they actually have the breakdown in german now that I look.
http://en.wikipedia.org/wiki/Grammatical_tense