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