A computer retrieval system includes memory which stores instructions for (1) receiving a user query in a natural language, (2) identifying text strings from an associated knowledge base, each containing at least one expression (word or multi-word expression) which is matched with at least one expression of the user query, (3) refining the user query by representing at least a group of the text strings as a navigation tree in which each of a plurality of selectable nodes comprises an expression representative of at least one syntactic unit in at least one of the identified text strings and wherein relationships between syntactic units are used in structuring the nodes of the navigation tree, and (4) retrieving text from the knowledge base related to the refined query. A processor executes the instructions.