What are some important vocabulary words to know when reading the book Hatchet?

Gary Paulsen's Hatchet is considered 5th- to 8th-grade reading level. As a student reads, the student may find some new, advanced words to learn to improve vocabulary and reading-comprehension skills. A few words worth getting to know are amphibious, grimacing, and instinctive.

We find amphibious in Chapter 5, soon after Brian's plane has crashed into the lake. Brian is eagerly anticipating being rescued. As he thinks about being rescued, the narrator reveals Brian's following thoughts:

Yeah, they would probably come today.
Probably come in here with amphibious planes.

The narrator also defines for us exactly what an "amphibious plane" is in the rest of the sentence: "small bushplanes with floats that could land right here on the lake." An amphibian is an animal that can live both on land and in water; therefore, anything that we call amphibious, like a plane, has the capability of being both on land and in water.

We find grimacing in the opening chapter as the narrator describes the pilot beginning to have a major heart attack:

Brian turned again to glance at the pilot, who had both hands on his stomach and was grimacing in pain, reaching for the left shoulder again as Brian watched.

Based on this description, we can tell that to grimace is to make an extreme, even ugly facial expression that shows pain or any other intense negative emotion.

Instinctive is another important word because, all throughout the book, Brian realizes he must control his instincts, by thinking carefully before he acts, in order to survive. However, by Chapter 18, while trying to retrieve the survival pack from the plane, a pack he'd forgotten all about for the past two months, Brian has a fright that nearly causes him to drown:

Too much. Too much. His mind screamed in horror and he slammed back and was sick in the water, sick so that he choked on it and tried to breathe water and could have ended there ... . It was instinctive fear more than anything else, fear of what he had seen.

Anything instinctive relates to instincts; instincts are natural impulses we are born with, such as the impulse to eat when hungry or to run when scared. Animals usually act upon their instincts.