Euphemisms are typically words or sayings that make something sound better than it actually is. In Fahrenheit 451, take the title of firemen for example. In our society, that brings up images of brave, heroic men that save people's lives at great risk to themselves. However, in Montag's society, firemen bring destruction, misery, great harm, and even death at times. But, the title "fireman" sounds so much nicer--it's a euphemism for the dark works that these men perform. Another euphemism is the "Fun Parks" that the teenagers go to. That brings to mind harmless carnivals, a family atmosphere, and lots of clean, safe fun. However, as Clarisse describes, in reality at these parks, they
"bully people around, break windowpanes in the Window Smasher place or wreck cars in the Car Wrecker place with the big steel ball."
So, their "Fun Parks" are more about violence and destruction than fun. But, their society has labeled them as fun to make those activities more acceptable.
Pathos refers to emotional appeals in the book. Any time there is something in the book that pulls at your heartstrings, that is pathos. For example, when the firemen release cats in the station only to be hunted and killed by the hound--pathos. When we learn that Clarisse was most likely killed--pathos. When Montag has memories of his mother and the good times they had--pathos. We feel in those moments; Bradbury is appealing to our soft sides.
Ethos refers to any discussion of ethics, or human rights and universal truths like happiness and freedom. Take a look at Beatty's speech to Montag for examples of this--he talks all about how as a society, we forfeited our rights and freedom in the name of political correctness and laziness. And colloquialisms can be found any time the people talk amongst themselves; for example, Clarisse mentions the kids playing "chicken" and "knock hubcaps." Those are local slang phrases for games that the teenagers play.
I hope that those thoughts helped; good luck!