1 A to-infinitive clause is often used after an adjective to mean 'in order to'. The adjectives often indicate that something is important or necessary, or any adjective may be used in this way with have to, must, or need.
Lips need to be bright to be noticed.
A car is essential to reach those wide open spaces.
The true Queensland is a land where you must be strong to survive.
A tripod is always useful to steady the camera.
2 A to-infinitive clause following an adjective is sometimes used to say that an action shows that someone has a particular quality. The adjective is used with must or have to.
Football clubs must be mad to pay millions of pounds for foreign players.
3 See also Sections 77 and 78.