"It is sad to leave."
Translation:Es triste partir.
I asked the same thing a few months ago. http://duolingo.com/#/comment/66399 The short answer is "leave" has more than one meaning in English(leave a location(partir), leave/drop off/end something/someone(dejar)). Neither Spanish word covers both of them. If the context was the person was breaking up with their boyfriend then maybe "dejar" should be allowed.
The answer is that you are understanding the purpose of ser in Spanish as a verb of duration rather than a verb used to identify "being in essence". The speaker of this phrase is identifying one thing (leaving) as being equal to another thing (sad). Because the speaker is claiming that leaving (it does not matter if it is always true or only true of this particular leave taking) is sad, that essence will not change. The person, however, is only sad in that moment because of the leaving. That is why estar is used to describe the speaker; the speaker is sad, but because of the leave taking, not because they themselves are in essence sad.
If it helps, here is what this difference signifies. Imagine this speaker thinking of this leavetaking in twenty years. While they will still describe the leaving as a sad event, the speaker has experienced a wide range of emotions over that time.
They list POSSIBLE translations. You have to pick out which one is the correct translation for the specific context of the sentence. Sometimes the correct translation isn't even listed in the possible translations. I gave your question a thumbs up. It is a perfectly good question.
Only thing I can think of is that the statement does not concern the temporary emotional state of a person, but rather is a statement that the act of leaving is "always" sad. So if I said I was sad to leave, it would be "Estoy triste partir" because it is about how I feel at the moment.
But I desperately need some more astute comment on this. (see the comment by jindr004 above along the same lines)