Why is Negative Evidence important in UG?

Negative Evidence

‘Negative evidence’ refers to the wrong or ungrammatical expressions. Negative evidence is a part of Chomsky’s universal grammar theory. According to him the input is degenerate in the sense that it does not usually contain ’negative evidence’ ,information from which the learner could work out what is not possible in a given language. Speakers proficient in a language know what expressions are acceptable in their language and what expressions are unacceptable. The key puzzle is how speakers should come to know the restrictions of their language, since expressions which violate those restrictions are not present in the input. This absence of negative evidence—that is, absence of evidence that an expression is part of a class of the ungrammatical sentences in one's language proves that a language is not learnable from input only. There are two kinds of negative evidences such as overt and covert.

Overt negative evidence is unavailable to a child because caretakers react to the truth value,not from,of children’s utterances and rarely correct the ungrammatical speech.Covert negative evidence is also unavailable ,since all that learners hear is grammatical utterances. 

In order to understand the negative evidence we can study the following two examples:
For example, in English one cannot relate a question word like 'what' to a predicate within a relative clause (1):
 (1) *What did John meet a man who sold?
Such expressions are not available to the language learners, because they are, by hypothesis, ungrammatical for speakers of the local language. Speakers of the local language do not utter such expressions and note that they are unacceptable to language learners. 

We can also study the following two sentences to learn about the negative evidence.

1,We gave the book to the girl.
2,We explained the answer to the girl.
Apparently these two sentences have the same surface structures,but whereas (1) contains an indirect object and can be rewritten as (3),
3,We gave the girl the book.
(2) contains a prepositional phrase and cannot be rewritten as (4):

4,We explained the girl the answer.

How does the child find out that ’give’ takes an indirect object and ’explain’ a prepositional phrase?How does the child discover  that (4) is ungrammatical?

One possible answer is that the adults correct the children , but the research does show the different thing. It seems logical to assume ,therefore , that there must be some innate principle which prevents the child from producing sentences like (4).