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 all scenarios
Complete the true ending
Obtain all light orbs
Complete Tomoyo Sakagami's scenario
Achieve a 64 hit combo with Tomoyo
Complete Nagisa Furukawa's scenario
Achieve glorious victory in the baseball scenario!
Complete Sunohara's ending
Complete Toshio Koumura's scenario
Complete Fuko Ibuki's scenario
Change class to Fuko Master
Complete Kotomi Ichinose's scenario
Complete the Sunoharas' scenario
Complete Kappei Hiiragi's scenario
Complete Kyou Fujibayashi's scenario
Complete Ryou Fujibayashi's scenario
Complete Yukine Miyazawa's scenario
Obtain a light orb
Jet Saito makes his appearance
Complete the gamebook, "Welcome to the Heartbeat☆Academy"
Complete Misae Sagara's scenario
Complete the prologue