There are 6 acts, if not performed in Salah, the Salah is invalidated and this is irrelevant whether the omission is intentional or by mistake. Even Sajda Sahw cannot compensate an omission in one of these fard. We have explained them in this link “6 Fard Acts during Salah omission of which invalidates the Salah”. Moreover, there are fourteen wajibaat in prayer. If any of the Wajib is omitted by an error in Salah, then it requires rectification. The procedure laid down by the Shariah to rectify the mistake is that a forgetfulness prostration shall be performed. If Sajda Sahw is not performed, Salah becomes invalidate and need to be repeated. It should also be noted that, if any person knowingly committed any mistake by leaving any wajib act in Salah then there is no Sajda sahw and he must repeat his prayer. The rule for the rectification of errors is equal to men and women.[irp]
Recitation in Fard Salah: It is wajib to fix first 2 Rakaat of Fard Prayers for recitation. There must be a recitation of Quranic verses in the first 2 units of fard prayer.
Recitation of Surah Al-Fatiha: It is necessary to recite Surah Faatiha in all the units of every Salah. Though, it is Sunnah to recite Surah Fateha in the third and fourth Rakaat of any Fard prayer.
Recitation after Surah Al-Fatiha: It is wajib to recite any surah or a single lengthy verse or at least 3 small verses of Holy Quran after Surah Al-Fatiha in the first two units of Fard prayers and in all the Rakaats of Sunnah, Wajib or Nafal Prayers.
Surah Fateha must be reciting first: The recitation must be in such an order that Surah Al-Fatiha must be recited before any other recitation.
Qauma: It is a standing posture after Ruku. A Person must stand straight after Ruku. It is Wajib act.
Jalsa: It is a necessary act to sit between Sajood (prostrations). A person must do one Sajda and then sit for a moment and then do the other Sajda. It is wajib to sit and remain still for a moment between the two prostrations
Qaa’dah Oola: It means to sit in the second unit (Rakat) of Salah to recite Tashahudd. A person is sitting as long as he recites Tashahudd. It is a Waajib act. Prayer does not end here, after reciting the Tashahud, you arise to stand up for the 3rd unit.
Tashahudd: It is wajib to recite Tashahudd in Qaa'dah Ooola and Qaa'dah Akheerah both. It is to be noted that there is the difference between Qaa'dah Ooola and Qaa'dah Akheerah. Qaa'dah Ooola is sitting in 2nd Rakat of prayer to recite tashahudd and it is Waajib whereas Qaa'dah Akheerah is an obligatory act Fard to sit in the last Rakaah of Prayer so long that you could recite whole Tashahudd to end the prayer.
Concluding Salam: It is a necessary act to say salam at the end of the prayer. Every Salah should be ended by saying Salam with the words “Asalam-o-Alaikum Wa Rahmatullah”. You must say salam at the end of the prayer, if not then you have to repeat your salah.
Loud Recitation: There are different rulings for the recitation of different prayers. It is necessary that Imam should recite in a loud voice in Fajr, Maghrib, Esha, Jummah, Eidain and Taraweeh prayers. Whereas he should recite Zohar and Asr inaudibly and silently. If a person is praying lonely or individually whether man or a woman, should recite in a voice to that level that he or she could hear it.
Takbeer of Qunoot: It is necessary that in the 3rd and last Rakaat of Wit'r to say takbeer and then recite Qanoot. There is an additional Takbeer in wit’r prayer’s last Rakat after which Qunoot is recited.
Six (6) additional Takbeers of Eid: It is Wajib to say extra takbirs in both Eidain Salahs.[irp]
The sequential order of Salah: It is wajib to have a sequence in prayer. There must be an order such as first do Qiyam, then Ruku and then Sujood (prostrations). It should be done one after the other and in order.
Ta'deel ul Arkaan: It means to perform Salah timely and all the Arkaan of prayer such as Qiyam, Ruku, and Prostrations with contentment. These must be done in a good way. It is necessary to do these arkaan calmly. It shouldn't be like just running and rushed to finish the prayer.
Source: Central Mosque