If you forgot something that is a pillar of prayer such as Al Fatiha, the bowing, the prostration etc, you have to cancel that Rak’ah and consider what was after it in its place and offer another Rak’ah in its place. After you finish your prayer, you have to offer two prostrations and another salam.

What should I do if I forget to do sujud or ruku and later I realise that before completing the prayers?