I would like to know why adjectives such as "in love"("enamorado") or "in pain"("adolorido") write in that way and what is the name of this kind of words. Thank you all in advance!!
"In love" is not an adjective. It is a prepositional phrase, just like "in England", or "after lunch".
It is true that it is most often used as a complement, as in "He was in love", and that adjectives can be used in that way, as in "He was happy". But that does not make it an adjective. Nor does the fact that you might use an adjective to translate it in another language.