1. He was excited to watch the game.
2. He was excited watching the game.
(Which one is grammatical? Are both OK? What is the difference between them?)
Both are OK, but they have slightly different meanings.
#1 - This implies that he would be watching the game soon (isn't watching it yet) and is really looking forward to it.
#2 - This means that he is excited about the game while he's watching it. Be sure to put a comma after "excited" to set off the participial phrase.