Dry flies are typically used when insects are hatching and floating on the surface of the water. Trout will rise to the surface to feed on these insects, making dry flies an effective choice. Nymphs, on the other hand, imitate insects that are still in the larval or pupal stage and are not yet on the surface. Nymphs are typically fished below the surface using weight and indicators to detect strikes. In general, if you see rising fish, use dry flies. If you're not seeing any surface activity, nymphs may be a better choice.