Assalamu Alaykum Ustadh, How do the fuqaha determine which acts are sunnah and which acts you must do in Salah (ie it is an obligation or pillar)?

For an in-depth answer, study Level 1 in the academy and finish it, and also Level 2. You will then know in depth how we determine whether something is an obligation or not in all aspects of the religion. As for your question regarding Salah (prayer), one of the main Hadiths from which the scholars derived what is obligatory in the Salah and what is not is the Hadith known as the "Hadith of the person who prayed badly." In this Hadith, the Messenger (sallallahu alaihi wasallam) told the man to repeat his Salah, and then he taught him how to pray: 1. Make Takbeer. 2. Recite the Fatihah. 3. Bow into Ruku, and ensure you have tranquility and remain in Ruku. 4. Rise and say, "Sami' Allahu liman hamidah." The same principle applies to the entire Salah: you must have tranquility and remain in each position for an appropriate time, rather than moving too quickly. In Arabic, this is called Al-Tuma'neenah, which means tranquility and remaining in each position for an appropriate amount of time.