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Ethanol in the presence of an acid, gets protonated in the first step, as hydroxyl group present in ethanol acts as a base and abstracts the proton and gets converted to good leaving group, that is water. In the next step, the anionic part of an acid acts as a good base and abstracts the proton from hydroxyl so as to neutralize the positive charge on oxygen which leads to formation of ethanol as the product.

Ethanol in presence of a base, undergoes acid-base reaction in the first step as acid-base reactions are the fastest, thus, base abstracts the proton from hydroxyl group of ethanol converting it into ethoxide ion which is a good nucleophile, and in next step, ethoxide ion attacks at the proton of an acid, and forms ethanol.