Universal grammar can be simply defined as a set of universal innate principles of grammar shared by all languages.That a child will not be able to learn its mother tongue without a set of innate principles ,because the linguistic data it is exposed to are too poor is the central point of the UG.The theory given by the American linguist Chomsky, attempts to explain language acquisition in general, not describe specific languages.The key features of the UG are given below.
UG consists of different kinds of universals.Chomsky identifies two types:substantive and formal.Substantive universals consist of fixed features such as the distinctive phonetic features of which sounds are made or syntactic categories such as noun,verb,and object.Formal universals are more abstract.They are statements about what grammatical rules are possible in languages.
Formal and substantive universals are constrains and therefore delimit the options by setting parameters which must then be fixed according to the particular input data that the child obtains.
So,the principles and parameters which can be defined as a framework in hunan brain make UG possible. UG holds a set of principles and parameters arranged into modules such as Binding Theory and it is a computation system that ranges from the component of Phonological Form to the component of Logical Form ,that is ,from ’sound’ to ’meaning’.
The rules the child learns can be unmarked or marked.The universals that the child learns form the core grammar and the distinctive features are termed as peripheral.
To sum up, there are some innate universal principles without which a child cannot master his mother tongue.But the input data are also a must as the input triggers the LAD.