Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(Then, even after that), after you worshipped the calf, (We pardoned you), left you alone and did not exterminate you (in order that ye might give thanks), so that you might give thanks to Us for pardoning you.
Then We pardoned you after that, your grotesque deed and vile transgression, through your repentance upon Moses's return to you, [so that you might be thankful] so that you might be grateful for the grace of My pardon by visualising that grace as coming from the Gracious One and [so that you might] prepare to accept the self-disclosure of the attribute of Grace. According to another interpretation [it means]: 'And We appointed for Moses the heart upon his becoming attached to the body and veiled from his people, the spiritual powers, for the forty nights during which his corporeal structure was being created; then you took to worshipping the calf of the animal soul, [Moses] as the infant, after his absence in the state of childhood; then We pardoned you after that, worship by the real maturation [of Moses] and the self-manifestation of the light of the heart upon your disengagement, so that you might be thankful for the grace of My granting you success in [attaining] that disengagement and My preparing [for you] of the means for your perfection by your following of the path of My attributes.
Allah then said, "Remember My favor on you when I forgave you for worshipping the calf.'' This happened after Musa went to the meeting place with his Lord at the end of that period which was forty days. These forty days were mentioned in Surat Al-A`raf, when Allah said,
(And We appointed for Musa thirty nights and added (to the period) ten (more)) (7:142).
It was said that these days were during the month of Dhul-Qa`dah plus the first ten days in Dhul-Hijjah, after the Children of Israel were delivered from Fir`awn and they safely crossed the sea. Allah's statement,
وَإِذْ ءَاتَيْنَا مُوسَى الْكِتَـبَ
(And (remember) when We gave Musa the Scripture) means, the Tawrah,
(And the criterion) that is that which differentiates between truth and falsehood, guidance and deviation.
(So that you may be guided aright), after escaping the sea, as another Ayah in Surat Al-A`raf clearly stated,
(And indeed We gave Musa ـ after We had destroyed the generations of old ـ the Scripture (the Tawrah) as an enlightenment for mankind, and a guidance and a mercy, that they might remember (or receive admonition)) (28:43).
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
When after their escape from Egypt the Israelites entered the Sinai Peninsula, Allah invited the Prophet Moses to Mount Tur for forty days and forty nights to give him Laws, Instructions and Commands for the guidance of the people who had recently been liberated from slavery. (For details, please see Exodus, chaps. 24 to 31). It should, however, be noted that the Tur, referred to here, is not the modern port of Tur on the eastern shore of the Gulf of Suez.
Cow-worship was very common in Can'an, Egypt and the neighbouring countries. When the Israelites degenerated and became slaves of the Copts after the death of the Prophet Joseph, they learnt this evil practice too from their rulers along with other evils. (The incident of the calf-worship is given in detail in Exodus, 32).
Then We pardoned you after that so that you might be thankful. The quickness [with which God] pardoned such a grave sin indicates the lesser weight of those who were pardoned. The words of God most high testify to that [in addressing the wives of the Prophet] Whoever of you commits manifest indecency the chastisement shall be doubled for her [33:30]. The children of Israel worshiped the calf so God most high said “Then We pardoned you after that.” To the community [of Muḥammad ﷺ] He said “and whoever does an atom's weight of evil shall see it” [99:8].