In "Fahrenheit 451" why do you think Bradbury would introduce Clarisse before Montag’s wife?
He introduces Clarisse before his wife in order to have a stark contrast to his wife. If you compare descriptions of the two women, they are like total opposites, and Clarisse represents the happiness that Montag is missing in his life, the happiness that Mildred obviously doesn't feel. Whereas Clarisse has a "soft and constant light" to her, Mildred is instead "cold, like a body displayed on the lid of a tomb." Where Clarisse seems to shine "your own light to you...to your innermost trembling thought," Mildred "felt no" emotion did not care whether her breath "came or went, went or came."
So, the two women are as different as can be. Introducing Clarisse first puts Montag in a frame of mind to ponder his own happiness. He gets home after his meeting with her wondering if he really was happy, and just as he does, he discovers his wife has attempted suicide. It emphasizes the fact that he--and Mildred--are not happy at all, and that somehow, this Clarisse girl is. Brinigng Clarisse onto the scene first makes the discovery of his wife even more tragic, and provides a good contrast.
I hope that these thoughts help; good luck!