Some suffixes for the word "sad" include:

sad + -en = sadden (meaning: to cause someone to be sad; to make unhappy)

sad + -en + -ed = saddened (meaning: past tense of "sadden")

sad + -ness = sadness (meaning: the feeling of being sad; sorrow)

sad + -er = sadder (meaning: comparative of sad, "more sad")

sad + -est = saddest (meaning: superlative of sad, "most sad")

sad + -ly = sadly (meaning: in a sad manner; with sadness)

There aren't many prefixes for sad:

un- + sad + -en = unsadden (meaning: to relieve sadness; to cheer up)

un- + sad + -en + -ed = unsaddened (meaning: past tense of "unsadden")