Praise be to Allah, and may peace and blessings be upon the Messenger of Allah. To proceed:
Divine decree (Al-Qadar) is Allah’s determination of everything that occurs in the universe, according to His prior knowledge and wisdom. Belief in it includes four matters:
Belief in Allah’s knowledge: That He has known everything from eternity and forever, both in general and in detail. Nothing concerning the actions of His creation, nor anything concerning His own actions, is hidden from Him, Glorified is He.
Belief in the writing: That Allah, the Exalted, wrote the decrees of all creation in the Preserved Tablet fifty thousand years before He created the heavens and the earth.
Belief in Allah’s will: That everything that occurs in the universe occurs only by the will of Allah, the Exalted. Whatever He wills happens, and whatever He does not will does not happen, whether it concerns His own actions or the actions of His creation.
Belief in creation: That all created beings are created by Allah, the Exalted, including their essences, attributes, and movements. He, the Exalted, said:
“Allah is the Creator of all things.” [Az-Zumar: 62]
Whoever believes in these four matters has believed correctly in divine decree.
No. Belief in divine decree does not mean that a person is compelled to perform his voluntary actions. Rather, he has a will and ability by which he can choose to do something or refrain from doing it, and therefore he is held accountable for his choices.
Allah, the Exalted, says:
“So whoever wills may take a way back to his Lord.” [An-Naba’: 39]
And He, the Exalted, says:
“So fear Allah as much as you are able.” [At-Taghabun: 16]
A person can distinguish between what he does by his own will, such as walking, and what happens to him without his choice, such as trembling.
However, the servant’s will and ability are not independent of Allah’s will. Allah, the Exalted, says:
“For whoever among you wills to be upright. And you do not will unless Allah, Lord of the worlds, wills.” [At-Takwir: 28–29]
Allah, the Exalted, is the One who created the servant, his ability, and his will. Nothing occurs within His dominion except by His knowledge and will. Nevertheless, the servant genuinely chooses his voluntary actions, and reward and punishment result from his choices.
In summary: Belief in divine decree means believing in Allah’s prior knowledge, His writing of the decrees, the execution of His will, and His creation of everything, while affirming the servant’s will, ability, and choice without them ever being outside the will of Allah, the Exalted.
And Allah knows best.