"It is sad to leave."
No. Most of time, to+verb is just translated as the verb in infinitive in portuguese, that is, verb ending in ar, er, ir (in this case, partir)
further examples: I want to leave = eu quero partir / I need to go = eu preciso ir / I like to eat a lot = gosto de comer muito / I have to wait = tenho que esperar
Thanks for your examples,helpful,really. Another question is What's the difference between"para"and"por"?
Hmmm, hard to explain. It depends on the context. But, a simple rule, para = for / to; por = by. That will help you many contexts.