只有the time的时候用when,如：
I will never forget the time when I spent with you.我永远也不会忘记和你一起度过的时光.
如果是有序数词the first,就只能用that了,在定语从句中,如果先行词有序数词,形容词最高级,不定代词,或是表示唯一概念的“the very,the only"时,或是先行词既有人又有物时,关系词只能用”that".
所以应该是:It was the first time that I had been to Beijing.