IRKAAN (PILLARS INSIDE SALAAT)
The pillars of salaat are five and they are what are considered fard in salaat. The person who leaves off any one of these five things whether it is done intentionally or unintentionally will render his salaat null and void.
Qiyaam (standing)
Salaat is not accepted sitting if you have the power to stand.
Standing is fard in fard and wajib salaats, it is not fard to stand in nafil salaats. It is permissible to sit when you have the power to stand in nafil salaats.
Qiraat (recitation of Quran)