Had Allah said...
Had Allah said, “When We said to him”, it would have implied that the hearers of this verse were present at that sublime station and could be addressed to by Allah - after all, the audience has a direct relationship with the speaker.
Therefore, Allah referred to Himself in the third person, cutting the connection between Himself and the hearers of the verse, showing that at the level where He spoke with Ibrahim, no one else was present; the conversation between Allah and Ibrahim was a confidential matter veiled in secrecy. In short, it was a talk between two close friends which others were excluded from. “I submit myself to the Lord of the worlds”.
As mentioned above, the preceding phrase shows that Allah bestowed His grace exclusively on Ibrahim and enhanced his rank by this friendly confidential conversation. But Ibrahim knew how to speak in Divine presence; he was a servant of Allah; he should not forget the dictates of humility; it was a sign of his excellence, of his humbleness, that he did not start talking with Allah in a friendly way, did not consider himself as worthy of that exclusive proximity, of that sublime friendship.
These verbs are used when a man or a thing submits to another thing, in such a way that the first never disobeys the second, never goes against it. Allah says: Yes! whoever submits himself entirely to Allah... (2:112); Surely I have turned my face, being upright, wholly to Him Who originated the heavens and the earth... (6:79) . It is with the face that one turns towards someone. So far as Allah is concerned, the whole being, the whole existence, of the thing turns to Him.
When a man surrenders to Allah, he obeys and accepts whatever comes to him from Allah - the creative matters like the measure and the decree, as well as the legislative ones like order and prohibition. As men differ in degrees of their submission to Divine Decrees or legislations, so does their Islam.