Hi, nohaypan. I agree with you completely. I understand that "below" is one of the Spanish translations for "debajo de" but when it isn't legitimate for the given context/phrase/sentence, I don't think Duo should put it as an "alternative translation" because it is just NOT. Oh, well. I'm reporting it.
For those posting about wondering what context "The pig is under the table" could reasonably have, this might be a reference to the movie "Casanova", which has an excellent Spanish dub, by the way.
There's a scene in which we think one of Casanova's lovers, who has been hiding under the table, is about to be discovered, but when his other lover looks, it's now just a pet pig (she accidentally created a fashion for pet pigs, on account of a strategic gambit she played using her alter ego).
Here is the scene:
There are MANY differences and it takes time to learn the differences. The quick way of distinguishing between estar and ser is 'temporary vs permanent', estar=temporary, ser=permanent, however, this isn't the most reliable way of remembering it because there are a lot of exceptions. There are many other ways of learning the differences, these links will help give you a start: http://spanishobsessed.com/spanish-grammar/ser-vs-estar/