  To illustrate how parameter setting works, consider a simplified  example (discussed in more detail in Chomsky 1990:644-45). All  languages require that sentences have subjects, but whereas some  languages (like English) require that the subject be overt in the  utterance, other languages (like Spanish) allow you to leave the  subject out of the sentence when it is written or spoken. Thus, a  Spanish speaker who wanted to say that he speaks Spanish could say  Hablo español (leaving out the first personal pronoun yo) without violating the rules of Spanish, whereas an  English speaker wanting to express that thought could not say  *Speak Spanish without violating the rules of English: to  speak grammatically, he must say I speak Spanish. The  parameter-setting model accommodates this sort of difference by  proposing that there is a ‘Null Subject Parameter,’ which is set differently in English and Spanish speakers: Spanish speakers  set it to ‘Subject Optional,’ whereas in English  speakers, it is set to ‘Subject Obligatory.’ How? One  proposal is that the parameter is set by default to ‘Subject  Obligatory’ and that hearing a subjectless sentence causes it  to be set to ‘Subject Optional.’ Since children learning  Spanish frequently hear subjectless sentences, whereas those learning English do not, the parameter setting is switched in the Spanish  learner, but remains set at the default for the English learner.  (Roeper and Williams 1987 is the locus classicus for  parameter-setting models; Ayoun 2003 is more up-to-date; Pinker,  1997: ch.3 provides a helpful, non-technical overview.)