  Take a child learning the grammar of her language, L.  Figure 2 represents the 5 possible relations that  might obtain between the language generated by her current  hypothesis, H, and that generated by the target grammar,  L. (v) represents the end point of the learning process: the learner has figured out the correct grammar for her language. A  learner in situation (i), (ii) or (iii) is in good shape, for she can easily use the pld as a basis for correcting her hypothesis  as follows: whenever she encounters a sentence in the data (i.e., a  sentence of L) that is not generated by H, she has  to ‘expand’ her hypothesis so that it generates that  sentence. In this way, H will keep moving, as desired, towards  L. However, suppose that the learner finds herself in  situation (iv), where her hypothesis generates all of the target  language, L, and more besides. (Children frequently find  themselves in this position, for example, they invariably go through  a phase in which they overgeneralize regular past tense verb endings  to irregular verbs; their grammars generate the incorrect *I  breaked it as well as the correct I broke it.) There,  she is in deep trouble, for she cannot use the pld to  discover her error. Every sentence of L, after all, is  already a sentence of H. In order to ‘shrink’  her hypothesis — to ‘unlearn’ the rules that  generate *I breaked it — she needs to know which  sentences of H are not sentences of L — she  needs to figure out that *I breaked it is not a sentence of  English. But — and this is the problem — this kind of  evidence, often called ‘negative evidence,’ is held to be unavailable to language learners.