Unless
Definition 1 of 3
Pro Tip 1/3
Conditional Permission
Understand 'unless' as setting a condition for permission, often used by parents or authorities. You can't have dessert unless you eat your vegetables.
Pro Tip 2/3
Implied Negative
'Unless' implies a negative scenario; it suggests that the action in the main clause isn't expected to happen. He won't text you back unless you apologize.
Pro Tip 3/3
Futurity Context
Use 'unless' to discuss future possibilities that depend on a condition not currently met. They can't start construction unless the permits are approved.