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