Uso del infinitivo: teoría

1. Como sujeto para hablar de un hecho concreto:
Ej.: It was a mistake to say it to him.
2. Detrás de algunos adjetivos y adverbios:
Adjetivos: afraid, amazed, angry, busy, delighted, difficult, disappointed, easy, glad, happy, hard, impossible, lucky, possible, ready, sad, sorry, surprised, unable, able, fast, high, low,...
Adverbios: fast, high, low, slowly,...
Ej.: I'm glad to hear from you.
3. Detrás de ciertos verbos:
Afford, agree, appear (seem), arrange, choose, decide, expect, fail, hope, learn, manage (arreglárselas), need, offer, plan, prepare, pretend (fingir, pretender), promise, refuse, regret, seem, swear, threaten, volunteer, want, wish,...
Be about (estar a punto de), would like/prefer/hate/etc., turn out (resultar, salir), be up to (you), leave it up to (you), do (your) best (hacer algo lo mejor posible, dar lo mejor de uno mismo para algo), make up (your) mind (decidirse),...
Ej.: Elena has made up her mind to go to university.
      It's your decision = I leave it up to you to decide.
      It turned out to be a great decision. (Resultó ser una estupenda decisión)
4. En la estructura verbo + objeto + infinitivo:
Advise, allow, ask, convince, encourage, expect, forbide, force, help, invite, order, permit, persuade, remind, tell, teach, try, want, warn,...
Nota: algunos de estos verbos van seguidos de gerundio si no llevan objeto.
   Ej.: advise, allow, permit.
          I advised him to talk to his mother.
          I advised going to the cinema.
5. Detrás de ciertos nombres (decision, attempt, ability, plan, promise, etc.)
Ej.: You must make an effort to pass your exam.
6. Detrás de adjetivos acompañados por enough y too. 
Ej.: You are too young to drink alcohol
      He isn't tall enough to reach that shelf.