Question

difference between Dew and frost

Answer 1

True frost forms when the temperature is below freezing. The moisture goes straight from a gas to a solid. If the moisture goes from a gas to a liquid and then to a solid then the result will be frozen dew. You can tell the difference between frost and frozen dew by looking at them.

Answer 2

frost is formed by freezing of water vapours that is deposited in saturated air
whereas dew are the tiny drops of water that form on cool surfaces at night.
Frost is observed as ice crystals mostly on grass. It basically forms dew, when it  freezes because the air temperature falls below the level necessary for freezing.