beneficial
The definition for beneficial is "Helpful." Used in a sentence: there is no doubt that eating vegetables is beneficial to one's health.
extant
The definition for extant is "Currently existing." Used in a sentence: Some consider Clint Eastwood the greatest extant American director.
meticulous
The definition for meticulous is "Extrememly careful regarding details." Used in a sentence: The meticulous detective found every shred of evidence at the crime scene.
naive
The definition for naive is "Lacking worldy wisdom." Used in a sentence: The naive Simon assumed that everyone was as simple and kind as he was.
preclude
The definition for preclude is "To rule out in advance." Used in a sentence: "Having cake now will preclude dessert later," said Amber's mother.