An obligatory part is waived if one forgets, and it can be compensated for by doing the prostration of forgetfulness (sujood al-sahw). It may be useful here to mention the pillars of prayer and its obligatory parts, followed by some of its Sunnahs, based on the book Daleel al-Taalib, which is a well known summary used by Hanbali fuqaha’.
1 – The pillars of prayer, of which there are fourteen, as follows:
2 – The obligatory parts of prayer, of which there are eight, as follows:
3 – The Sunnah words of the prayer, of which there are eleven, as follows:
4 – Actions which are Sunnah, which are called postures:
There are some differences of opinion among the fuqaha’ regarding some of these issues; what is regarded as obligatory by some is regarded as Sunnah by others. This is discussed in detail in the books of fiqh.
And Allaah knows best.
Source: IslamQA & Sheikh Assim AlHakeem
The Islamic Ummah