A natural language interface for a computer system restricts inputs from a user to those which generate valid input sentences. The user selects a next input word from a menu showing which words are valid. When an input is selected, a parser uses a predefined grammar to determine which words are valid continuations after the selected word, and whether a complete sentence has been entered. Valid next words are presented to the user in menu form so that a next selection can be made. If an ambiguous complete sentence is entered, the interface allows the user to select the desired meaning therefor. The grammer can be context free grammar, and can use an associated lexicon to define elements therein.