in my first semester, my teacher had a little saying that helped with the te-form conjugations of godan (u) verbs. the ichidan (ru) verbs are very easy to remember, because there's only two possibilites for the verb ending (iru, eru)
うつる:って (u tsu ru tte)
むぶぬ:んで (mu bu nu nde)
く:いて (ku ite)
ぐ:いで (gu ite)
す:して (su shite)
so if you see a verb like "korobu," you can tell that it's te form is "koronde." If you see "koronde" for the first time, the dictionary form is either "korobu," "koromu" or "koronu." It takes some getting used to, but with some practice, it becomes natural and easy to conjugate verbs on the fly without having to think about it.