Here is the full list of all 21 Mad Father achievements. It takes around 5-6 hours to unlock all of the achievements on Windows.
See the "If" Ending
Unlock a secret event...
See Bad Ending 1
See Bad Ending 2
Give the pendant to mother's corpse
Read mother's love letter in the second week
Give the dog a bone
See the True Ending
Unlock image: "Charm Of Happiness"
Rescue the corpse from prison
Receive a present from the mysterious plant
Get chased by father
Examine the specimens in the second-floor basement
Rescue the large man
Read Maria's diary
Help the hungry boy
Read the New Text on Anatomy
Help the eyeless girl
Talk to the muttering old man
Unlock image: "It's Behind You"
Unlock image: "Mother's Portrait"