First room: The gimmick of this game is that 'green' things have a retroactive effect on the game. Try undoing after getting the green sword, paying attention to your stats changing (or rather, what stats DON'T change).
Using Greenality #1: You can always meta-restart (ESC) to change your mind, so experiment by using it on everything you can think of and building a mental model of what the consequences are. It is not required to do the absolute most optimal use of Greenality #1 to beat the game, but you may enjoy discovering how some uses of it are better (or even strictly better) than others.
Making it from Greenality #1 to Greenality #2: Both remaining Greenalities are heavily guarded. Try to decide if you're killing the monster to get to the Greenality, or if your goal is to reach other items lying around the dungeon and to discover their properties through experimentation. This is as much a rule discovery puzzle as it is an optimization puzzle, expect to be in both 'modes' at different times.