The difference between Sad and Unhappy

When used as adjectives, sad means feeling sorrow, whereas unhappy means not happy.

Unhappy is also noun with the meaning: an individual who is not happy.

check bellow for the other definitions of Sad and Unhappy

Sad as an adjective:

Emotionally negative. Feeling sorrow; sorrowful, mournful. Appearing sorrowful. Causing sorrow; lamentable. Poor in quality, bad; shameful, deplorable; later, regrettable, poor. Of colours: dark, deep; later, sombre, dull.

Examples:

"She gets sad when he's away."

"The puppy had a sad little face."

"It's a sad fact that most rapes go unreported."

"That's the saddest-looking pickup truck I've ever seen."
Sad as an adjective (obsolete):

Sated, having had one's fill; satisfied, weary.
Sad as an adjective (obsolete):

Steadfast, valiant.
Sad as an adjective (obsolete):

Dignified, serious, grave.
Sad as an adjective (obsolete):

Naughty; troublesome; wicked.
Sad as an adjective (slang):

Unfashionable; socially inadequate or undesirable.

Examples:

"I can't believe you use drugs; you're so sad!"
Sad as an adjective (dialect):

Soggy (to refer to pastries).
Sad as an adjective (obsolete):

Heavy; weighty; ponderous; close; hard.

Examples:

"sad bread"

Unhappy as an adjective:

Not happy; sad.
Unhappy as an adjective:

Not satisfied; unsatisfied.

Examples:

"An unhappy customer is unlikely to return to your shop."
Unhappy as an adjective (mostly, dated):

Not lucky; unlucky.

Examples:

"The doomed lovers must have been born under an unhappy star."
Unhappy as an adjective (mostly, dated):

Not suitable; unsuitable.