A generative grammar is a set of rules that tries to include all examples of correct language and predict how these will be formed.
The tree structure is important in the context-free generative grammar model. It describes phrases in terms of constituent grammatical parts.
In the classroom
Generative grammars are of limited use to learners and are not meant to be a guide to how to use language. Learners looking for more information about grammar can be supported by grammar usage books, which show how structures are used in language, and by prescriptive grammars, which describe rules.