‘Iz’, which means ‘when’, is a term used for the past tense. Every occurance requires a time and place for it to happen in, and ‘iz’ here refers to such and such a time, meaning, “remember the time We saved you”
The word ‘iz’ is repeated in the Holy Qur’an a number of times and it is a high form of language. Look at Allah’s U word:
﴾إِلَّا تَنصُرُوهُ فَقَدْ نَصَرَهُ اللَّهُ إِذْ أَخْرَجَهُ الَّذِينَ كَفَرُوا ثَانِيَ اثْنَيْنِ إِذْ هُمَا فِي الْغَارِ إِذْ يَقُولُ لِصَاحِبِهِ لَا تَحْزَنْ إِنَّ اللَّهَ مَعَنَا﴿
“If you do not aid the Prophet – Allah has already aided him when those who disbelieved had driven him out [of Makkah] as one of two, when they were in the cave and he said to his companion, “Do not grieve; indeed, Allah is with us.” [Surah At-Tawbah: 40]