I'm also a new Chinese speaker (well, not that new in comparison... I've been studying it for 3 years now), but I'm pretty sure verbs themselves don't have a past tense. You add "le" (sorry, my computer doesn't display Chinese - the character is very simple, it looks almost like the number 3) after the verb. Or at the end of the sentence. I can't remember when to use either one.

Eg:
"Wo mai le yi ben shu" means "I bought a book."
"Wo kan shu le" means "I read a book."

I hope that helps... I'm not a native speaker myself, any natives out there feel free to correct me.