When, Al Quraan and Ahaadith differ which has precedence, why ?

Allah guarantees that Al Quran wont be altered until the day of the Judgement. So Al Quran gets the first priority.

Al Hadith is the example of how to implement Al Quran in life, and also teach us what is not described in Al Quran, such as shalat. Al Hadith is never contradict with Al Quran.

Soon after the passing of Muhammad pbuh, false hadiths are sprouting like mushroom after the rain, so if you see a contradiction in them, consult Al Quran first.

Ahaadith explained in detail of the verses in Al Qur'an. So do not follow hadith that is baseless.

"Among the people, there are those who uphold baseless HADITH (narrations, sayings, words, speech, opinions, statement, utterance, discourse, account, tale........), and thus divert others from the path of God without knowledge, and take it in vain. These have incurred a shameful retribution." 31:6

"God has revealed herein the BEST HADITH (narrations, sayings, words, speech, opinions, statement, utterance, discourse, account, tale.....); a book that is consistent and points out both ways (to heaven and hell). The skins of those who reverence their Lord cringe therefrom, then their skins and their hearts soften up for God's message. Such is God's guidance; he bestows it upon whomever He wills. As for those sent astray by God, nothing can guide them." 39:23