Adam Clarke on Matthew 6:16

When ye fast - A fast is termed by the Greeks νηϚις, from νη not, and εσθειν to eat; hence fast means, a total abstinence from food for a certain time. Abstaining from flesh, and living on fish, vegetables, etc., is no fast, or may be rather considered a burlesque on fasting. Many pretend to take the true definition of a fast from Isa 58:3, and say that it means a fast from sin. This is a mistake; there is no such term in the Bible as fasting from sin; the very idea is ridiculous and absurd, as if sin were a part of our daily food. In the fast mentioned by the prophet, the people were to divide their bread with the hungry, Isa 58:7; but could they eat their bread, and give it too? No man should save by a fast: he should give all the food he might have eaten to the poor. He who saves a day's expense by a fast, commits an abomination before the Lord. See more on Mat 9:15 (note).

As the hypocrites - of a sad countenance - Σκυθρωποι, either from σκυθρος sour, crabbed, and ωψ the countenance; or from Σκυθης a Scythian, a morose, gloomy, austere phiz, like that of a Scythian or Tartar. A hypocrite has always a difficult part to act: when he wishes to appear as a penitent, not having any godly sorrow at heart, he is obliged to counterfeit it the best way he can, by a gloomy and austere look.