Here is the full list of all 22 CLANNAD achievements. It takes around 50-60 hours to unlock all of the achievements on Windows.
Complete Sunohara's ending
Jet Saito makes his appearance
Change class to Fuko Master
Achieve a 64 hit combo with Tomoyo
Complete the gamebook, "Welcome to the Heartbeat☆Academy"
Complete all scenarios
Achieve glorious victory in the baseball scenario!
Complete the true ending
Obtain all light orbs
Complete Nagisa Furukawa's scenario
Complete Toshio Koumura's scenario
Complete Fuko Ibuki's scenario
Complete Tomoyo Sakagami's scenario
Complete Misae Sagara's scenario
Complete Kappei Hiiragi's scenario
Complete Kyou Fujibayashi's scenario
Complete Ryou Fujibayashi's scenario
Complete Kotomi Ichinose's scenario
Complete Yukine Miyazawa's scenario
Complete the Sunoharas' scenario
Obtain a light orb
Complete the prologue