When to use what trout fly

With trout flies you have to use the conditions at the river to pick which is the right fly. If the fish are feeding below the surface of the water then use a wet fly, one that is designed to sink. If you see the trout on coming to the river surface then use a dry fly to keep it floating on the surface. As for which trout fly to use? Use the fly that most resemble the bugs and insects at the river when your fishing. If after a while of using one fly does not yield results switch to a new fly and keep trying. Eventually you'll catch a nice river trout.