Spelling: British/American and Y > i

1. Notice that some words have a different spelling in American English:

 British spelling:                                                                 American spelling:
centre, metre, litre, theatre (-RE)                  center, meter, liter, theater (-ER)
neighbour, colour, honour (-OUR)               neighbor, color, honor (-OR)
travelling, traveller                                      traveling, traveler
programme                                                    program

Both spellings are correct, but you have to be consistent: if you write CENTER (American), you should also write COLOR, TRAVELING, THEATRE, etc. For obvious reasons, in your book you will find the British spelling. It would also be strange if your accent was not American but your spelling was!

2. When you add and ending (añades una terminación) to a word that ends in -Y, we usually chage -Y to -i-:
story > stories      spy>spies     hurry>hurries     easy>easier     hurry>hurried     happy>happily
However (=¡ahora bien!), we do not change -Y to -i- after a vowel letter:
boy>boys     play>plays, played    key>keys     stay>stays, stayed     buy>buys
Conclusión: recordad que se cambia o no depediendo des si antes de la -Y hay vocal o consonante. BOYS es fácil de recordar, así que de esta palabra podemos deducir la regla. Vocal delante de la -Y >>> no hay cambio.