1. If a person intentionally leaves one of the pillars of Prayer, such as reading surat al-Fatihah and sujud, then his Prayer is invalid.
2. If he forgets to read surat al-Fatihah or to perform other pillar of Prayer such as sujud, and he still in the same rak`ah (and doesn’t start reading in the next rak`ah), then he has to be back to do that missed pillar and continue his Prayer normally, and offer sujud as-Sahaw at the end of the Prayer before saying the salam.
3. If he forgot to read surat al-Fatihah or to perform other pillar of Prayer such as sujud, and he started reading in the next rak`ah, then the incomplete rak`ah is invalid and he has to consider the rak`ah he is doing to replace the other one and continue his prayer normally, and offer sujud as-Sahaw at the end of the Prayer before saying the Salam.
4. If he forgot to read surat al-Fatihah or to perform other pillar of Prayer such as sujud, and ended his Prayer, and then remembered after a long time, he has to repeat the Prayer.
5. If he forgot to read surat al-Fatihah or to perform other pillar of Prayer such as sujud, and ended his Prayer, and then remembered after a short time, he has to offer a rak`ah in the place of the incomplete rak`ah and to offer sujud as-Sahaw.