My gum got injured during dental impression.the da purposely hurt me with a wrong size tray.could that damage my tooth nerve & cause me gum infection?

Wound infection: I find it hard to believe that a dental assistant would intentionally try to hurt a patient. Having said that I also doubt that using the wrong size tray could harm a tooth and although it's possible for any wound to become infected, in a healthy individual the likelihood of it becoming infected would be extremely unusual.

The: Dental assistant tries to use the smallest tray that will do the job because an over large tray will extend too far back and make you wretch or gag. Often the contour of the bone under the gums is such that a very slightly too small tray will dig into the gum. (S)he was trying to make you more comfortable, not hurt you on purpose. At the most, your gum was scratched. It will heal quickly.