Praise be to Allah.
If a person recites another soorah before al-Faatihah, his prayer is valid so long as he recites al-Faatihah afterwards. But this is not what is prescribed in Islam and the one who does this is mistaken. One has to recite al-Faatihah first, then recite whatever one can of Qur’aan afterwards, because the Prophet (peace and blessings of Allaah be upon him) said: “Pray as you have seen me praying.” When he prayed, he recited al-Faatihah first, and did not change this sequence in any of the many prayers he prayed during his lifetime. It is obligatory to follow the Sunnah. And Allaah knows best.