We present the advantages of guided sentences composition for communicating in natural language with computers. We show how guidance can be achieved by means of the partial synthesis of sentences and describe our approach consisting in separating knowledge that comes under different levels of well-formedness and coroutining their treatment. This approach has led us to develop in Prolog a software, ILLICO, allowing for the conception of natural language interfaces with guided sentences composition. We present an application we have developed by means of ILLICO : KOMBE, a communication aid system for handicapped persons. 1 Guided Sentences Composition We may distinguish two kinds of communication with natural language interfaces : - a "free" mode : the user types sentences without knowing the limits of the interface but he hopes he will be understood. Trivial reality : user's freedom will always be the freedom the system will grant him. - a guided mode : the system guides...