To understand what he can do lets look at his skills: It's also important to note that each of his skills (except Transform) operates its cooldown seperatly for each mode (Melee or Ranged). The thing with Jayce is unlike most other character's set of 3 skills, he in fact has a 7-skill-long combo-chain, and each skill in the combo is a different ability. To counter Jayce you have to understand how he works, the best and shortest answer I can give you is, To learn how to counter Jayce - top lane, play Jayce - top lane.
