Unless I misunderstand the question, the difference is simple.
"Ambition" (ambizione) is a noun, meaning a "goal".. My ambition is to speak Italian..
"Ambitious" (ambizioso) is an adjective, meaning "wanting to get ahead" or "waiting to achieve a goal". He was ambitious at work, trying to get a promotion.
Neither of these words has anything to do with past tense, as they are not verbs.