السلام عليكم
I think قَدْ is added to the past tense to serve as an intensifier, as in to show that that event 'had' happend, so for the translation of this verse, I think it would be:
قَدْ جَاءَكُم مِّنَ اللَّهِ نُورٌ وَكِتَابٌ مُّبِينٌ
'(Indeed) From Allah, a light and a clear book HAD come to you.'
In this case, I feel that the verse is saying, 'Allah HAD sent enlightenment/knowledge to you - (but still you go astray?)'
Nurjahan, you mentioned something interesting - It didn't come to my mind that قَدْ could grammatically be a preposition. Could anyone confirm this إن ساء الله؟
* As a side point, something I found very inspiring was the grammatical vowelling of this sentence - The verb خاء (he/it came to) of which the object is كُمْ (you), but notice the subjects 'doers' here (marked by ضمّة) are actually the light and the clear book نُورٌ وَكِتَابٌ مُّبِينٌ! Allah is showing that his signs CAME to US, سبحان الله how eased is the guidance of Allah upon us, rather than using the verb بَعَثَ (he sent), it is as though - these personified signs CAME from Allah with a will to guide us to our Lord - BEFORE WE CAME to seek them! How eloquently said...It's soo beautiful!
مع السلامة