How do we know that “belief in qadar” is one of the pillars of faith? Is it mentioned in the Quran?

Divine determining is also known as qadar (as an Islamic term), destiny or fate. And the key concept to understand "qadar"is "pre-eternity".

Is destiny (qadar) already pre-determined? If yes, can’t we change our future mistakes with our acts, precautions or prayers (dua)? Is destiny fixed?

Is destiny (qadar) already pre-determined? If yes, can’t we change our future mistakes with our acts, cautions or prayers (dua)? Is destiny fixed?

“If human is free, then how divine determining is compatible with it?” “Is human the creator of his own actions or is he just a leaf on the wind?”

Allah gave the human beings a “will” that is free to make their own choices; differentiating and choosing the good or the bad.  If the human beings were doomed to their destiny, “free-will” would not be given to them.

Does man choose his spouse with his free will or does Allah predetermine his choice? Is the choise of spouse predestined?

Does man choose his spouse with his will or is it predetermined by Allah?

How much is the effect of human will on death? What is the share of destiny on our death time?

Is death time fixed or can it be changed? For example, by smoking do we shorten our life span? Or do we lengthen it by taking care of our health?