Those which are so essential that without them the Prayer cannot be considered valid, are called Fard, i.e. obligatory, mandatory or compulsory.
If Fard constituent parts are not carried out, the Prayer becomes null and void. However, in case a worshipper does not perform that part because he completely forgot about it, the Prayer will be held valid in the sight of Allah. If he had forgotten at the time but remembers it either during the Prayer or after the Prayer, or if someone reminds him of his omission, then he should rectify the omission by performing the function that has been missed and then perform the Sujud-us-Sahv, i.e. two prostrations by way of condonement. These compulsory functions are as follows:
Takbir-i-Tahrimah: To say Allahu Akbar to begin the Prayer.
Qiyam: The posture of Standing.
Ruku‘: The posture of Bowing down.
Sajdah: The posture of Prostrating. The above are the common features of every Rak‘at.
The Final Qa‘dah: The last long sitting position before ending the Prayer. This feature is not repeated in every Rak‘at.
Recitation of Surah Al-Fatihah: The recitation of Surah Al-Fatihah is also essential in every Rak‘at.
When the Imam is leading the congregation, the loud recitation of Surah Al-Fatihah is compulsory in the first two Rak‘at of the following Prayers:
Fajr Prayer, which comprises two Rak‘at.
Maghrib Prayer, which comprises three Rak‘at.
‘Isha’ Prayer, which comprises four Rak‘at.
Note: If the recitation of some portion of the Holy Quran is not done in the first two Rak‘at, the Rak‘at will not be invalidated, but on remembering this mistake, the two Sujud-us-Sahv become essential to validate the Prayer. If the Imam forgets to recite Surah Al-Fatihah aloud and also the additional verses of the Holy Quran in any Rak‘at, and he is reminded of this before he goes to the Bowing position, he should complete this function by reciting Surah Al-Fatihah as well as the verses of the Holy Quran and then go into Ruku‘. In this case no prostrations of condonement are necessary. However, if he has led the congregation into Ruku‘ and then remembers his mistake, the repetition of the function, i.e. the recitation of Surah Al-Fatihah and the verses of the Holy Quran, is not needed. Only two Prostrations of condonement are enough to make the Prayer valid.