In this paper we present a natural language interface architecture, which is used for accessing FAQ knowledge bases. Since one of the main obstacles to the efficient use of natural language interfaces is the high amount of required manual knowledge engineering, we provide an adaptive architecture to automate the acquisition of linguistic knowledge. We apply a machine learning module based on an XSE (eXtended Semantic Enumeration) tree to activate linguistic tests from semantic, syntactic, and pragmatic analysis during the traversal of the tree. The XSE-tree is built automatically based on past questions without the need for any additional linguistic knowledge.