Using the right word usually means picking striding vs lumbering when describing how a hero walks. But there are mistakes that are easy to make. Savvy readers will wince and even casual readers will know something is off. Here are some of the more common gotchas.
There, their, they're. "Look there! They're screwing their story." ("Look there (place)! They're (they are) screwing their (possessive--think mine!) story.")
Discreet, discrete. This is one of mine. "Be discreet about who you tell. Keep who you tell discrete." (Discreet is careful; discrete means distinct parts--remember ete keeps the ees distinct, separate.)
It's, its. This is bizarre because "its" is the opposite of the general possessive rule (Nixie's book). But "it's" is NOT possessive. Think yours, mine, ours--its. In this case "it's" is the contraction (it is). So it's really hard to know its possessions.