Complex sentence: The Constituent Assembly had not adopted a national anthem by the time Pakistan became independent, so when the flag was hoisted at the independence ceremony it was accompanied by the song, "Pakistan Zindabad, Azadi Paendabad".
Split into simpler sentences: The Constituent Assembly had not adopted a national anthem by the time Pakistan became independent. So when the flag was hoisted at the independence ceremony it was accompanied by the song, "Pakistan Zindabad, Azadi Paendabad".