What is the opposite of "Happy"?

Hello! Thank you so much for reaching out with your question. 

There are numerous words that could be used as the opposite of "happy", but ones most commonly used are "sad", "unhappy", "miserable". Here is an example of a sentence where all three of these options would work: "His dog had just died, and I could see how sad/unhappy/miserable he was". 

I hope that these options and this example help you.